I'm no lawyer, so I can't say which one is right here. But... I believe Frylock's argument is how it should be (well, except him claiming copyright over the stat blocks himself).
The way I see it, a stat block is analogous to a recipe, which can not be copyrighted. You can copyright "fluff" around the recipe, and possibly some flavorful prose within the recipe, but not the basic list of ingredients and instructions. For example, the recipe for a Pan-Galactic Gargle Blaster includes the instruction "Allow four litres of Fallian marsh gas to bubble through it, in memory of all those happy hikers who have died of pleasure in the Marshes of Fallia." The phrase about the happy hikers who died in the Marshes of Fallia would be protected, but not the part about allowing four litres of Fallian marsh gas to bubble through it.
Similarly, a stat block is just a statement of facts. Made-up facts, to be sure, but still facts. A goblin is weak, but nimble and sneaky. A grell has paralytic tentacles. A flesh golem has their strength replenished by lightning. The stat block is just a reflection of those facts.
I just looked through about half the stat blocks in Volo's Guide to Monsters (because I figured that would have a greater chance of having fluff in the stat blocks), and other than the occasional name mentioned (e.g. "Vaprak's Rage"), I don't see anything that's not just a statement of in-game facts. The stuff outside the stat blocks is definitely copyrightable (e.g. the stuff about a Frost Giant Everlasting One being the result of a frost giant eating a troll in a ritual dedicated to Vaprak intended to grant the giant greater strength and resilience), but not the stat blocks themselves.