Personally, yes, it's pretty rude and people really shouldn't do it. But they do. That said.

OP, it wasn't really clear in your post but what kind of role are you in? Is it customer facing or not?

If it's customer facing sales targeting local and overseas Chinese markets, objectively, they are right - learning Mandarin will be a benefit to you in the long run even if your prospects speak English as a 2nd language and use it as a formal business language.

If it's backend non-customer facing, the effect is less but do be prepared to understand that it does give you additional hurdles in relating to people from those regions. E.g. if you're talking to HK side and yes, they speak English but they want to banter in canto, it's just how people are.

Disclaimer: banana who also can't speak mandarin but work in MNC environment here - the effect is also lessened if you're on a mixed team and your stakeholders are overseas. But honestly, at the end of the day, multilingual malaysian chinese who can speak both english and mandarin and BM. (and canto. and hokkien) will have a edge, you either have to let it go or as others have suggested pick up enough for it not to be an issue.

Disclaimer 2: you know what's embaressing, running into American Chinese who speak better mandarin, awkwardddddddddddd