It was effectively stolen because the intent was to release documents that were not ready for public consumption. Is that a legal definition? No. It is a moral one.
I support freedom of information too. When information is being kept from the public because it contains evidence of government misdeeds, excesses, etc. or because it embarrasses officials with evidence of personal misbehavior, FOIA is great and critical.
This is not that. Evan Falchuk had no conceivable reason to FOIA that particular slide other than to short-circuit Boston 2024's vetting and technical review process and cast the bid group in the worst possible light. There's no evidence of corruption or misdeeds in it, and he had no reason to expect that there would be.
He can legally do it, but it's an @$$hole move, plain and simple.