Actually « extension » is more specific than TLD. .co.uk is a sold extension but it’s not a TLD, even though it works as what we usually call a TLD.

Maybe if you said "more general", but TLD is definitely "more specific" (which is why co.uk is not considered a TLD).

Referring to TLDs as extensions is a telltale sign of someone who is semi-technically literate enough to have heard of file extensions but not enough to know why it's incorrect when talking about domains, nor that it outs them by making them sound silly.