Whether the offset thumb actually exists or not, seems to be more a question of semantics than it is a question on reality. SOMETHING obviously does exist.
The way my drillings are measured off, it WOULD be an "offset" thumb, due to the grip center line being drawn perpindicular to the finger span line, then having to measure 3/8 inch laterally from that along my thumb span line to find the top center of my thumbhole. This works perfectly for me every time.
There is a drilling technique called CLT, or Center Line Transfer/Transposition, in which the thumb is "centered", then the finger holes are drilled at differing distances from that, giving the visual effect that the finger holes are the ones that are "offset".
Either style seems to be effective in producing holes in the proper geometry, with the difference being the reference point from which pitches are measured, which CAN make a difference.
My way works for me, and I already know what pitches work with this technique for me, so I stay with it for that reason.