I think that since this goy was not truly interested in being a gerr it was assur for Hillel to teach him torah. Therefore Hillel had to tell him “mai disony alach…” which is the foundation of his the 7 mitzvos. This is muttar to teach a goy.
I am still bothered about the mitzvos of arayos and ever min hachay which don’t apply under this rule.
It is also difficult to undestand that “Reachah=HBH” for a goy.