The definition of "Frontman" is Lead Singer…is a drummer who is the primary lead singer a frontman? Phil Collins? Don Henley? Peter Rivera (Rare Earth)?
Also, there are many bands, historically speaking, that had legit multiple lead singers…
Beatles (3); Eagles (4); Byrds (2); Pink Floyd (2); Fleetwood Mac (3); Little Feat (3); Jefferson Airplane (4); The Band (4); The Monkees (3); Doobies (3-4); The Clash (2); CSNY. Hell, The Beach Boys had 5.
For me, it is all about body language, basically saying "I'm the one in charge here" even if the aren't autonomous within the group, and between song banter with the crowd…Jagger; Daltrey; Stewart; Bon; DLR; Axl. I am more partial to a true frontman not playing an instrument (tambourine / maracas excepted).
There has to be some flair, some drama, some diva in there to be a true frontman, IMO…