I know that both of them means "on the contrary".
But I have seen that それどころか can also mean "as if it's not enough already", " In fact/ actually, and "not to speak of ".
Does 逆に have all these other meanings too?
逆 literally means "contrary" or "reverse", so it never means "in addition". But 逆に has a broader sense than "on the contrary", and it may be translated as "rather", "in reverse", "adversely", or "in reverse order" depending on the situation.
- Can "逆に" be used to say "in addition"? (this is your own question)
- Understanding what 逆に means in context with an emotion
Translating 逆に as "actually" is fine when it implies "contrary to what you may expect" or "contrary to what has been asked". For example, when someone asked "Are you busy recently?", a possible reply would be:
Actually, I have too much spare time.
Yes, actually I am very busy. (逆/それどころか cannot be used)
Xどころか is used to strongly negate what has been said and tell something more "extreme". The following "extreme" statement can be either something quite contrary to X or simply more extreme than just X. For example, if someone said "Hmm, this car looks expensive." at an auto show, a possible reply would be:
TL;DR: When something is a severe understatement, you can use それどころか but not 逆に.