No they are horror films with aliens, you could call them sci-fi horror but horror is the main component. Just like H.P. Lovecraft always used alien entities as the antagonists but you wouldn't call his stories sci-fi.
Predator (like Alien and The Terminator) is essentially a slasher film with an antagonist type often found in sci-fi. The Thing mainly combines body horror, psychological horror, and cosmic horror. You could say it has elements of science fiction, but it has less of those than The Thing from Another World or say the Andromeda Strain.
I love The Thing but I didn't pick its' antagonist here as it's not really a single antagonist, it's too non-traditional.