Note that no one...not Mohaupt, Thomanek or the Brits...yet understood in 1939-40 that rotating a HEAT device bollixes the jet formation. Both Brandt and German ammo makers sold their respective armies a 75mm HEAT round that, in a non-rotating test jig against proof-plates, delivered brilliant results, but mysteriously was very inconsistent and at its best much less effective in actual service.
Eventually the OTL Brits captured German HEAT cannon-ammo in Libya, and certainly it got their attention. But, the OTL Brits simultaneously were noting that the Germans sometimes seemed to be firing HE shells at them that would gouge away a small divot externally, but usually not penetrate. The OTL Brit ordnance engineers knew what a HEAT penetration looked like, from their own #68 development and testing, and few if any tanks were being battlefield-recovered with such damage. That was the first indication for the Brits that somehow the new German ammo-type wasn't working as intended. It took quite a while, though, for the Brits, and the Germans too, to independently realize what the real physics issue was.
So, in regard to the Brandt company's various advanced technologies being "overtaken by events"...yes, in OTL Brandt had far less impact than might have been the case, due to French procurement dithering and senior command fight-the-last-war-but-this-time-we'll-win idiocy, and due to Brandt's ineffective salesmanship resulting in the right Brits not knowing about Brandt's products. Brandt in OTL did sell significant numbers of their 37/25 APCR and APDS rounds to the French Army, along with either 75,000 or 150,000 rounds (sources differ) of their 50mm HEAT RG, and small numbers of their 75mm HEAT cannon round and the 75/57 APDS cannon round. But, the 37/25 rounds were not significant...those guns were just not powerful enough, and the tanks that carried them were too unreliable, to be relevant to the outcome. The 50mm RGs sat in metro-Paris warehouses because the Army hadn't yet developed the training program for them, and certainly no weapon could be issued without proper training. And, the 75mm stuff existed in tiny quantities relative to need.
In an alternate timeline, all the second line French divisions along the Meuse would have been amply provided with 75/57 APDS and updated direct-fire gunsights for their 75mm artillery pieces, and ordered to site and prepare them to engage attackers in direct fire. The 50mm HEAT RGs would have been broadly issued. The French still would lose, due to the multitude of other shortcomings of their actions and preparations. But, suppose Carden had met Edgar Brandt before the war began and Carden had convinced the Vickers Board of Directors to license Brandt's technologies, with Vickers joining with Brandt in trying to sell those technologies to the British Army, and Vickers de facto becoming a business partner of Brandt.
In OTL, when France was attacked, Edgar Brandt directed his engineers to depart for Britain to try to help the British (they eventually were accepted for war work, and helped develop the APDS rounds for the 6 pounder and the 17 pounder); and sent a complete set of engineering drawings of his best products to USA (but those drawings didn't reach anyone that understood them, and most were lost); and OKed and funded Henry Mohaupt's departure for USA (where he would be denied permission to work on any ordnance projects, due to not being a US citizen, even though USA was using his patent description to train engineers on how to do the work he'd originated!)
In the alternate timeline, Edgar Brandt, his entire senior staff and their families, along with the company's technical documents, would have decamped for Britain, aided by British diplomatic and Army "facilitators" arranged through Carden's Vickers contacts, and been set up in suitable offices and labs in Newcastle, quite close to Vickers' own facilities.
Then Britain would have quickly arrived at an understanding of why the #68 HEAT RG underperformed...its lack of standoff distance and its non-optimal cavity shape and liner, not it being "too small"...in fact, it was larger and heavier than needed, so shorter ranged. Thus British soldiers would have had highly effective HEAT capabilities two years sooner. And, Britain might have beat Germany to a Puppchen-like light-smoothbore-gun design to fire a 90-to-100mm fin-stabilized rocket HEAT round out to 300 meters or so, replacing the Boyes AT rifle on Carriers and in other light applications, and effective against anything the Germans would field throughout the rest of the war. And most significantly, Britain would have understood how to make medium-velocity cannons effective against tank armor, at least through mid-war, thus helping to fix much sooner all the inefficiencies resulting from the Cruiser/Infantry design split, and greatly supporting Carden's efforts to create Main Battle Tanks in their place.