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.