Even if you correctly format a column, excel will ignore that if it sees something that looks like a date. That's part of the problem: this isn't just automatic formatting, it's very aggressive and hard to turn off. Plus you cannot revert the changes excel has made: turning 01/02 is converted to an ibt in 46000 range. So reformatting the cell doesn't get the original input back, it just lands you with a bunch of ints.

Back in the day I actually wrote a function that would undo this for some sheets that people kept breaking...

This is not auto-formatting, it's deeper than that. It is not actually possible to disable Excel's date recognition, although you can re-change the format of a cell after the fact.

Excel is from an era when programs still catered to power users. Tools were made to have learning curves, ideally not particularly steep curves, but curves nevertheless. It wasn't expected that users would hit the app running, intuiting everything there was to know about the program in their first minute of using it.

The result is a rich deep program that users can grow into, rather than a shallow trivial program that optimizes for the noob experience and leaves power users out in the cold.

Auto-formatting seems like more of a noob feature than a power user feature. Lacking control over when it happens is definitely not a power user feature.

Treating SEPT1 as a date is behavior that will be correct for the vast majority of users, be they noob or experienced, and only incorrect for a very tiny minority of users who are doing things related to genetics. This sort of auto-completion feature is orthogonal to the noob/power user axis, except insofar as the user is expected to know how to circumvent this behavior if/when they need to.

The power user behavior would be to only do the conversion on cells that are typed as DATE datatype. But that would catch the noobs who don't know to set the cell type.