Actually on a second look the wordlist has 279,498 words. I think it was only partially loaded when I first checked. That bumps the math to log₂(279,498) ≈ 18.

Actually on a second look the wordlist has 279,498 words. I think it was only partially loaded when I first checked. That bumps the math to log₂(279,498) ≈ 18.1, so 19 guesses in the worst case and about 18 on average with perfect play