Yeah I could see that, since taking on AI teammates is also a way to handicap yourself. I think much depends on the scale of the map, like how many factions are involved, and also how long it takes the AI to complete a normal gameround, when you're only playing as a single faction among many. For example, in Iron War if you just play as Germany or Japan there can be a lot of down time between turns, compared to something like v5 where you'll be back up again pretty quickly.
It's also kind of curious for me to ponder what makes for a good/enjoyable experience playing against the HardAI in general, since it's never going to match PvP in terms of the challenge, even if you really juice the income bonuses. I think a big part of the appeal comes simply from that desire to paint the map you're own colors in new ways, or just to see a deep endgame situation that might be unlikely in PvP, where a human opponent would have already thrown in the towel a while ago lol.
Thinking about this stuff, I'm also reminded of other games that were primarily aimed at solo play vs the computer, and how they maintained engagement. Like say MTW 1, where you'd have stuff like Glorious Achievements (i.e. special victory conditions, where the player has to achieve specific goals within a certain timeframe for a given faction) or other randomizing events like rebellions or civils wars and such to mix things up on campaign. A&A style gameplay is of course a bit different than Total War, since the mechanics here are a lot more transparent. There's just not as much happening under the under the hood, to obfuscate what's going on with all that stuff. In a game like v5, giving the computer opponent starting tech advances can also work. You know like Japan starting with Super subs or shipyards or whatever, which is sorta Iron Blitz style I guess. Still I think stuff like special Victory Conditions, might worth exploring. In my view its better if the game/designer lays out some difficulty parameters or guidelines in the launch settings, since leaving it up to the player to invent a challenge for themselves ad hoc, is rather less satisfying somehow.