"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", isn't necessarily about to play basketball; they could be playing football. Similarly, someone who says "I'm involuntarily celibate" isn't 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 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.