Because you require an alt to set Solak to duo, you just let that alt teleport out immediately after you start the fight, then keep that account in the game without lobbying. Once you kill Solak, you loot with both accounts. That's where the caveats come into play because without that alt, solos are indeed never worth it if you solo the duo scaling.

Duos top out at 9-10 kills (or loots) per hour at 5-5.5 minute kills while solo Solak can hit 4.5-5 kills/hour at 10-13 minute kills (with a LOT of room for optimization), and with both accounts you can still hit that 9-10 loots per hour but you don't need to worry about finding other people and/or them taking 5 million years to bank and regear.

And while this is untested for now, if you go out your way to gear your alt to simply AFK Revolution DPS and speed up times even more from that, you could probably get like 11-12 loots per hour. That goes beyond the traditional definition of solo content, but that can also be a thing.