My theory is that it was a way to test the waters for it without committing to it. The only ‘tells’ that could indicate it being ‘April fools’ is it being released the day before and its title. Outside of those two things, nothing else indicates it being a joke.
My guess is WT was planning to implement infantry in some form, probably as a way to secure points and as a more active deterrent to spawn camping and CAS, and discussions naturally turned to playable infantry. While Enlisted was always intended to be WT with infantry, the reality is the game isn’t doing too well, and is basically a mode of WT that is arbitrarily its own game, at least at this point. Initially, it was its own thing, but over time, it’s become more and more like WT.
They probably weren’t sure how receptive people would be, so they ‘announced’ it in such a way where they could move forward with it if the interest was there, and if it received backlash they could drop it and say ‘April Fools’. Seeing as how it’s almost at a million views, and my personal anecdotal experience in seeing a lot of people excited at the prospect, I imagine that Gaijin is making plans to either absorb Enlisted into War Thunder, or kill Enlisted and implement Infantry into War Thunder.