Gerson is completely unrelated to Jevil outside of them both being Shadow Crystal bosses, yet his battle theme has the motif. It is very likely just a leitmotif for Shadow Crystal Bosses and not Jevil himself; the lack of it in Black Knife is likely due to how different the Knight is to the other Shadow Crystal Bosses.
The one theme unrelated to SCBs that has the motif, AIRWAVES, has the internal file name "spamton_dance," so it very likely was just intended to be a BIG SHOT remix and thus doesn't make the TWR motif less of a SCB motif; this is more plausible when you remember that the whole Mike room was a late addition for the Switch 2.
So, should we reframe whose motif it is and remove the part of Jevil's infobox saying it's his motif, and then add a section to the infobox of the Shadow Crystal Boss article saying it's their motif?
It’s still his leitmotif. Leitmotifs don’t necessarily need a name on them, but associated with the character. And as of now, Jevil has no other leitmotif associated with him.
I'd say there's no real need for the wiki to speculate on the meaning of motifs at all. It's best just to name them after whatever song they appear first and/or most prominently in. Though since currently two recurring motifs are attributed to TWR, it'd probably make sense to reattribute the first one to The Circus? I feel like as much as possible we should try to not name multiple motifs after the same song, and where it is really the best option (such as the three The Legend motifs) it'd probably be best to list them as seperate motifs (as "The Legend motif #1/#2/#3") for clarity on exactly which motif is being used.
TWR shouldn't be listed as a leitmotif because its meaning is disputable. For our purposes, it's a motif, and we name those after the first comprehensive appearance. Any other pages that imply that it's a leitmotif should be corrected since they're probably remnants from before the major revamp of the page.
I could argue even further that the B section of Gerson's quote actually references the instruments used in TWR, so it alludes to Jevil more than any of the other extra bosses.