Can you point to where you see that definition of Houston County's border? I didn't check primary sources before making edits, and the state of Alabama doesn't provide any comprehensive county boundary data that I can find, but the Houston County parcel viewer (the only semi-official county boundary I can find online) that I'm looking at now doesn't match your description of the county border along that stretch as far as I can tell, in fact it appears to be identical to the TIGER line that I added.

https://arcweb.houstoncountyal.gov/ParcelViewer/