Maybe "indoctrination" was a poor choice of word here. The problem with this maxim is that it welcomes moral relativism.
This can be bad on the assumption that whoever is exposed to the maxim is not a proponent of "virtue ethics" (I use this as a catch-all term for various religious ethics doctrines, the underlying idea is that moral truths are given to people by a divine authority rather than discovered by studying human behavior, needs and happiness). In this situation, the maxim is an invitation to embrace ideas that aren't contradictory to one's own, but that live "outside the system", to put them on equal footing.
To make this more concrete, let's suppose the subject of child brides. Some religions have no problem with marrying girls of any age to men of any age. Now, the maxim suggests that no matter what your moral framework looks like, you should accept that under some circumstances it's OK to have child marriages. But, this isn't a contradiction. There's no ethical theory that's not based on divine revelation that would accept such a thing. And that's why, by and large, the Western society came to treat child marriages as a crime.
Contradictions are only possible when two parties agree on the premises that led to contradicting conclusion, and, in principle, should be possible to be resolved by figuring out which party had a faulty process that derived a contradicting opinion. Resolving such contradictions is a productive way forward. But, the kind of "disagreement" between religious ethics and "derived" ethics is where the premises are different. So, there can be no way forward in an argument between the two, because the only way the two can agree is if one completely abandons their premises.
Essentially, you can think about it as if two teams wanted to compete in some sport. If both are playing soccer, then there's a meaning to winning / losing, keeping the score, being good or bad at the game. But, if one team plays soccer while another team is playing chess... it just doesn't make sense to pit them against each other.