I don't think you're giving them enough credit. They probably know about Steam, but when venturing to a new platform there is always risk. Epic Games Store exclusivity is, as always, risk mitigation.Good-Feel is made up of former Nintendo employees who wanted to break free from the Corporate culture. And yup it seems very likely they were convinced to join EGS.
I'm worried they might not release on Steam because they don't have enough knowledge.
Obviously there are some high profile games that would've done well on Steam going to EGS for exclusivity, but as time has passed those are in the minority. Even in the bad old days when Epic was flexing much harder than they are now, you could take a good look at a significant portion of the games that signed exclusivity deals and a lot of them were from developers just starting out, or indies whose last game was a flop, and extrapolate that they signed on the dotted line because Epic were offering them cashflow certainty.
I don't think this is a case of "Japan being Japan" or not understanding the PC market - this is just about cashflow certainty. They made a game that released over a year ago on Switch and didn't get enough reviews to earn it a Metascore, so I imagine the sales are bad. The PC version could've already been in the works, and the developer has no confidence it's going to sell, or they specifically made a PC version to pitch to Epic for exclusivity in order to help bankroll their next project.