It was def possible to beat chess engines back in the day using things like the horizon effect, by directing it to these positions as much as possible. For the slow thinkers, playing at correspondence speeds obviously helped
Same thing with old Poker Bots like PokerSnowie, it had plenty of weak links in the rarer spots you could navigate it, plus the too stringest sizing constraints.
A current AI bot with quick real time nodelock capabilities might just be unbeatable HU though, but as others mentioned, 6/9 handed the entire table composition might still be the most important aspect in determining your EV, not how tough the bot is.