I think time is part of the answer here.
It appears from posts in other threads that TurboTax added the "we won't allow you to efile if you use Simplified AMT" hard stop around March 19th. If you filed before that, it's entirely possible that you could have efiled, if you avoided the other 1116 gotchas that Foxbat has detailed, even if you had Simplified AMT on. I haven't heard anyone say they've been able to efile with Simplified AMT more recently.
Seems entirely possible to me that TT's 'fix' on March 19th was unnecessary - that they misunderstood the issue and the Simplified election wasn't even really a problem at all. But we'll never know, will we?
