I think that everyone has different perceptions of 'good' and 'bad' and 'harmful' and 'harmless', so if you are wanting a hard-and-fast absolute definition of which behaviours constitute abuse and which do not, then you are going to be disappointed.
To a degree almost all 'challenging' or 'aggressive' or 'assertive' or 'conflictual' behaviours except the most explicitly harmful (eg punching someone in the face) could qualify as either abusive or non-abusive depending on the context, at least in a one-off situation.
For example, if your husband talked over you during an argument as a one-off, pretty much nobody would call that abusive. But if he regularly talks over you during arguments as a way of controlling the conversation and getting his way, then this might reasonably be seen by many people as a low-level abusive behaviour, or even high-level depending on how aggressive he is about it.
However – as your friend says – even if most people would agree that consistently talking over your partner in arguments is a form of abuse, that doesn't mean that every individual (including, probably, the person doing the talking-over) will agree that the behaviour is abusive. Because people do have different standards for what they consider acceptable. You just won't get all people to come to an agreement on it.
So, the important thing is to recognise that your own standards are your own standards and just because they don't match those of another individual, it doesn't mean you're right and they're wrong, or you're wrong and they're right. It means you're not well-matched in terms of standards. And at the end of the day, that's all you need to know.
Your ex said you were "too sensitive" and he wasn't abusive; you feel that you were not too sensitive and he was abusive. Who is objectively right or wrong is less important than the fact that you didn't like the way he treated you and he didn't think there was any problem with the way he treated you.
Maybe you are quite a sensitive person but being sensitive isn't a necessarily personality fault in itself, it just means you work best with a partner who is also quite sensitive.
Maybe you are not especially sensitive but your ex sees you as being overly sensitive because he is extremely insensitive. It's also possible also that you are sensitive to a fault and over-reactive but if this is the case, you will probably find that you have the same problems you have with your ex with everyone else in your life. If you don't, the answer is more likely to be that you fall within the range of normal sensitivity but your ex falls below it.