I think there’s only one reasonable interpretation of “spells only do what they say they do” in this case. The idea that this means that the grease can’t be ignited puts a weird privilege on the property “flammable”. In the real world, pretty much most things can be lit aflame, and grease is usually one of them.
Saying “the grease spell doesn’t say the substance is flammable, so it is fireproof” is like saying “wall of stone doesn’t say that the wall is visible, only that it’s solid, so you can’t see it”. That’s…. a route to madness. We use a common sense, English language interpretation of what “stone” is, and the same should apply to “grease”.
Now, it may be that “flammable” is special, and there is a non-written rule that this is a property that nothing has unless otherwise stated. But, that seems to be exactly what Crawford is cautioning about in this tweet — “There aren’t secret rules.” There definitely isn’t a written rule about this.
So, what’s the reasonable interpretation? The grease may indeed be flammable, but it’s not so specially flammable as to cause significant extra fire damage — if it were, it would say so. I’d rule that it either burns in a flash that does no damage, or minimal damage like the non-magical damage from a lit torch as a weapon — save or take 1 point of fire damage.