How doesnt it make it plausible? This is a timeline in which they went down the nuclear path more than we did. By the Great War they had already managed to make fusion happen in basically anysize and keep that fusion going...
But hopefully the T-51 series will be able to run a lot longer, but i wouldnt be supries if it runs just as long as the other. PA by lore is basically OP, but its all about balance.
So you can either have power armour that runs for basically ever in the T-51s case, but have it less tanky to balance it. Or do as what seems to have been done and make it a tank, and just require recharging if you will. The fact we can buy them has me a lot more happy though, and I am sure you will be able to get them from the brotherhood. HOPEFULLY!!!- we will be able to craft them at some point though.