The HOA can not restrict purchase sure, but can specifically target PoC for "violations" while ignoring the white households who are committing violations. This has the effect of working to force these folks out and maintaining the racial status quo of the HOA.

In the 70s (I think) and before, HOA were definitely active gatekeepers though against PoC, and then after that with new laws passed, moved to these more "subtle" methods.