The two situations that you just described are completely different to Slash's solo album. The first being that it's Gilby. The casual fan didn't know who Gilby was back in the day. A lot of casual fans mistaken Gilby for Izzy. Gilby wasn't a GNR member. Regarding Anxious Disease, that was on West Arkeen's album. As I said with Gilby, the casual fan didn't know who he was. I do remember on the HTGTH website along with Jeff's ( first website dedicated to GNR back in 95), there were hardcore GNR fans who thought that Axl and Slash would reunite when they collaborated on this song.
With that being said, I did read a rumor on Facebook that Metal Chestnut is a collaboration between Axl and Slash. If it's that's great. If it's not that's cool too. However, the expectation that Axl will be on the album takes away from Slash. There's a reason why he didn't have GNR and SKMC members on the album.