It's not an actual EGS requirement, it's a poorly done implementation of the EOS online service. It basically just makes a generic online ID for your steam account, you don't have to actually make an account with email etc or install EGS client. Normally games that use EOS just automatically do that in the background.

Today they've admitted fault and are looking into doing something, likely fix it to use the latter method.

It wasn't malicious like Arrowhead / Sony, just Sandbox being a classic "how to into pc" japanese dev. I'd be outraged over actually forcing an epic account too, but I'll give them the benefit of the doubt on this one.