Why are you talking about solo Bono? Can you understand something recorded by ONE member of the band not being considered band productivity, while something recorded by ALL of them is? No, you can't, apparently.
All Howie B did was produce a track and shout "Elvis" over and over again. He probably "played" more on Pop with all the "decks and loops". Pavarotti sings one section of one song. There's one lead vocal by Holi. That's not much compared to the rest of the music on there.
Is Zooropa now a side project because Johnny Cash sang a vocal? How about Rattle and Hum, which has a Dylan backing vocal on a song he co-wrote, Dylan playing the organ, a snippet of a homeless guy performing, a gospel choir, a horn section, and B.B. King singing and playing guitar. Well, I guess we can remove that album from the 80's output now.
Do you see how ridiculous your logic is? It's beside the point though, because as The Tourist and I (and others) have said over and over again is that we are trying to account for the band's productivity, and whether you like it or not, for much of 1994/1995, ALL the members of U2 were writing and recording music. That's it, case closed, please stop trying to bend this discussion to fit your narrow (and obtuse) view of things.