Doh. This is even simpler. They are Chinese characters. They don’t exist as Kanji or official radicals. There’s no Japanese version, not that I found anyway. For streamlining purposes, WaniKani calls anything it wants to use in mnemonics a “radical” and has an item and lesson for each one. Most of them are just Kanji, so when the page displays it, it’s just a text character for that Kanji. A few are “real” radicals which also have text characters.
Some don’t actually exist as real text anywhere so WaniKani actually needs to use image files instead. This is why the gun and leaf radicals are actually PNG files. The Keisei Semantic-Phonetic script can’t even display them for that reason.
These ones do exist as Chinese text characters, but they’re not used in Japanese. I searched a Kanji database and it said they’re not in the records. (Actual radicals are in the records.) Instead of bothering to mock up a PNG imitation, WaniKani just uses the Chinese character.
TLDR: They appear as Chinese characters because they are Chinese characters.