Nothing you quoted invalidates my point. PHB doesn't say it's not flammable, it has historically been flammable, and people don't follow/agree with Sage Advice. Simple as that. It's not a BG3/PHB difference, it's a BG3/Sage Advice difference.
Plus, if we followed the Sage Advice reasoning, there are a TON of things players and DMs pull that are "against the rules" because he's taking a 'only things clearly defined are allowed' stance in this instance when it's not taken in any other instance. Can a player recover caltrops? The PHB doesn't say they're "recoverable" so the Sage Advice logic says no but a practical application would be that of course a player could spend 5-10 minutes picking them up.
I changed my text to leave Grease more open ended. I'm going to test the spell soon, as it might just be that much more of a utility thing being flammable. Either way, this list of differences isn't to poo poo devs or to champion Rules Lawyers...it's just to give people a guide to things that might not be what they expect, and if something really sucks, we'll be honest about it.
I really appreciate you speaking up about Grease though, I've never seen anyone use it in a P&P game as flammable, but that doesn't mean that it wouldn't be a better spell for it.
Well, I have always gone with, if it does not state it can do something, it probably can't. Look at the Web Spell, main use is to restrain creatures but also can be set on fire. If grease is flammable, why did they add that for Web but not add it for Grease?
"The webs are flammable. Any 5-foot cube of webs exposed to fire burns away in 1 round, dealing 2d4 fire damage to any creature that starts its turn in the fire."