“Partner” being the main definition is perfectly fine, but to completely ignore the second definition is, to me, a huge mistake.
While almost all dictionary entries for 相手 have “opponent” as a definition, some dictionaries even put that definition before “partner,” and sometimes “opponent” is the main definition! (I’d list some dictionaries, but I’m sure you have your own, trusted sources that you can check!)
Colloquially, I’ve seen 相手 used to mean “opponent” way more than any other definition (albeit in manga and games where that definition is more likely to appear.)
To not teach “opponent” as a definition of 相手 is to confuse Japanese learners who will inevitably encounter that definition, and it punishes those who have encountered the word before, or outside of, WaniKani.