They didn´t need to design it, only refurbish those old T-45d. They built the Prydwen from nothing but pieces found on the old airbase after all, a giant airship that can carry Vertibirds. Compared to that, how hard would it be to improve some systems and armored pieces of an already existing design of PA?
Well, exploring that would be a lot more interesting than most of what we see in F4 I guess ... anyway, it's not the first time that Bethesda doesn't give a crap about what happend in F1 or F2 or what the loore says about something.
I can't help it but feel constantly like Bethesda is recycling a lot of content that was actually meant to dissapear afte Fallout 2.
Take the Super Mutants and FEV as best example. In F1 they have been your enemy. By the time of Fallout 2 they already start to dwindle, same for the Ghouls, the BoS and a lot more. And now? It's like everyone has some FEV lying around somewhere, I guess we will have to accept that Super Mutants are the Orcs/Ogres of Fallout now.
I have no clue how much "old" Stuff Van Buren was supposd to contain, but it is pretty obvious that they also included a lot of new factions, where Fallout 4, and even more so Fallout 3 is taking "old" factions and creatures that have been on the brink of fading just to transport them across the whole continent ... for reasons I guess?
In the intro video you see a soldier wearing a T-60 at the frontline while it´s being repaired. There are also two soldiers protecting the access to Vault 111 wearing it. And the whole BoS is equipped with them. Hardly a just "being tested" or "just released" armor, if they could get it to frontline soldiers and even use it on soldiers guarding Vaults.
That's actually pretty common in war though. See the Sherman and M10 tank hunters with 76mm canons rushed into service after 1944 to battle new German armor, or the conversion of Shermans to M36 with 90mm guns for that matter, or sending the M26 Pershing heavy tanks across the ocean right before WW2 ended in Europe to get some field testing, maybe 20 units on the European theater actaully saw some action.
Of which at least one incident became a very famous battle. Pretty sad and very fascinating footage for anyone interested in WW2 by the way. However the Pershing, or better known as the M48, an upgraded vesion of the Pershing designated as medium tank, didn't really started to make an real impression before the Korean War in the 1950s.
Or, the Germans for example, they lost even more than 70% of their Panthers on their first battle, simply because they rushed them into service to be ready for the fighting in 1943 at the Kursk salient. The Tiger 1 was also used as early like 1942, even though it didn't really made it's debut in larger numbers before 1943, they have send at least 4 Tigers to the Leningrad front for field testing in late 1942.
So just released armor and designs in service next to older designs is definetly not rare in a prolonged war.