It's just that McLane didn't really mean to imply that the flesh was from a big worm. He said from a wormlike creature.
6 seconds sound clip from the Attack of the Crab Monsters (1957) movie soundboard.
You can hear this line at 00:11:31.146 in the DVD version of the movie.
Quote context
[...]
- Well, it's getting very late. Why don't we work out tomorrow's schedule and then get some sleep.
- What about that five foot nightcrawler?
- Well, excuse me for being so stupid, me and that book you're reading.
- We weren't laughing at you, Hank.
- You know, it might've been a sea worm. They've been known to grow much longer than five feet.
- Nothing in my experience leads me towards McLane's worm theory. Nothing.
- You would know better than I. But why, I ask, did the writer stop in the middle of a sentence?
- Yes, why, Karl? Unless something really unusual happened, I'm sure he would've finished.
[...]