The difference is that hobgoblins are explicitly evil and Lizardfolk are neutral. The game’s implied play style is not to randomly kill non-evil races. Even for evil races there is typically a reason to raid their home (I.e., they kidnapped someone, they are raiding passing caravan’s, etc.).That's the third older D&D adventure I can think of where the premise is "the PCs should murder a bunch of innocent people and then have to do a minor adventure to make everything okay again." I think it's probably best to modify an adventure like that in some way, because most players don't appreciate being punished or judged for accepting a game's implied play style. Generally, you want the players to feel comfortable buying into "the text says these hobgoblins are raiders and it's the excuse for a fun dungeon crawl" if you're ever going to run dungeon crawls, and pulling a bait and switch erodes trust. So it's best to just say up front, "these lizardfolk aren't evil, don't kill them" or strongly signal it.