I'm absolutely NOT disagreeing with this; in fact, I agree. But to relate ancient experience...
In a prior life some 20-odd years ago, wen I worked for a company that had an Enrolled Actuary and the EA always attended the EA annual convention, this issue came up. At the time, the IRS representative opined (of course, this was unofficial) that it was ok establish the plan on, say, 1/1 even though the employer didn't exist until later.
Back then, all plans other than standardized were submitted for determination letters, and we had plans like this approved with nary a question.
Haven't done this for a long time, and wouldn't do it now without some sort of clear guidance.