It's true that the language enunciation is very close for Cantonese speakers, but it's the phrases used that are quite different.

Mainland Mandarin vs. Taiwan Mandarin for example ~ there are a ton of phrases, especially for modern things like "credit card", "take out"..., that people use different words/phrases for. This is also the same for HK Cantonese vs. Cantonese used on the mainland where there was no British influence.

It's like UK english vs. US english.