Here are some excerpts from a RC site discussing 'sect':
"In the Acts of the Apostles it is applied both in the Latin of the Vulgate and in the English of the Douay version to the religious tendency with which one has identified himself (xxiv, 5; xxvi, 5; xxviii, 22; see xxiv, 14). The Epistles of the New Testament disparagingly apply it to the divisions within the Christian communities. The Epistle to the Galatians (v, 20) numbers among the works of the flesh, "quarrels, dissensions, sects"; and St. Peter in his second Epistle (ii, 1) speaks of the "lying teachers, who shall bring in sects of perdition". In subsequent Catholic ecclesiastical usage this meaning was retained"
"With the rise of Protestantism and the consequent disruption of the Christian religion into numerous denominations, the use of the word sect has become frequent among Christians. It usually implies at present disapproval in the mind of the speaker or writer. "
Emphasis supplied.
As I wrote before, I'm not willing to concede the language. If they want a different definition than that in the dictionary it's their problem. It's the same as the nonsense of claiming they are the one true church.