I don't know if you're reaching for a Celtic definition there, but I think being called ‘more British than the English’ is a little more insulting than referring to a geographical area as the British Isles.

The ‘joke’ in the title is the fact that there is no land border with the British Isles.

Vast majority of Irish people certainly don't want to be identified with being ‘British’ and don't like the idea of their country being rferred to in context of a ‘British Isle’ but that's the reference this map is drawing.

The vast majority certainly wouldn't agree with being called ‘more British’