Sexual frustration is a hyponym of inceldom in the same way that "basketball" is a hyponym of "sport". Someone who says "i'm going to play sport", is not necessarily about to play basketball; they could be playing football. Similarly, someone who says "I'm involuntarily celibate" is not necessarily sexually frustrated; they could simply be seeking a romantic relationship, love, marriage, might solely be longing for offspring or simply want the honor of having arm candy. They also may accept their involuntary celibacy and not be 'frustrated'. It is a big mistake to conflate sexual frustration with incelibacy because someone who is sexually frustrated could be sexually active, whereas someone who is involuntary celibate is by definition the opposite - sexually inactive, which according to most inceldom-experiencers in the incelosphere is a key characteristic of inceldom.
Unlike sexual frustration which by definition requires agitation, the Donnelly study found that only 35% of incels expressed dissatisfaction, frustration, or anger about their lack of sexual relationships.