A constructio kata synesin (or constructio ad sensum in Latin) means a grammatical construction in which a word takes the gender or number not of the word with which it should regularly agree, but of some other word implied in that word. It is effectively an agreement of words with the sense, instead of the morphosyntactic form.
Here, the plural pronoun they refers to the singular noun band. One can think of the antecedent of they as an implied plural noun such as musicians.
Such use in English grammar is often called notional agreement, because the agreement is with the notion of what the noun means, rather than the strict grammatical form of the noun. The term situational agreement is also found, since the same word may take a singular or plural verb depending on the intended emphasis of the speaker or writer; so:
Notional agreement for collective nouns in very common in British English. It is less customary in American English, but may sometimes be found after phrases of the type "a collective noun of plural nouns". Fowler (3e, 1996) cites for example