if not ... then what is it?; it must be ...; it is definitely ...
A rhetorical expression following a noun or adjective. It strongly asserts that something is exactly that thing, often with a nuance of 'what else could it be?' or 'it's nothing other than ...'. The tone can be emphatic or explanatory.
If this isn't love, what is it? / This is definitely love.
His actions were nothing other than courage.
A more formal and emphatic expression meaning 'nothing other than'. でなくて何だろう is more colloquial and rhetorical.
A fixed expression composed of the negative でなくて (from だ) + 何 (what) + だろう (speculative). Literally 'if it is not ..., what is it?', used rhetorically to assert that something is exactly that thing.