I don't think that's the arguement, it's not
"Arc Sys is involved with the game, So we are not gonna add in Sol or Ragna"
It's that Arc System was decided not to get a Fighter in FP2 (whether Sol or Ragna), and the River City spirits is a hint that that was the conclusion to Arc Systems potential attempts at a Full fighter in FP2
It's the same logic with how Bethesda probably isn't getting a fighter in FP2 because of the Vault Boy costume, same thing with something like the Ubisoft costumes
Now because the notable series of Guilty Gear and Blazblue weren't the actual franchises that were spirited, and that Spirits aren't as much of a selling point as Mii costumes or Fighters are, there could be the argument that perhaps they could still be in, but I understand if people thik that Arc Systems getting any form of content around FP2 lifecycle, implies they weren't chosen for a Challenger pack spot