That is not the case.
There was a new law….
The AA make it simpler reading and say the following:
Unless you've registered your vehicle as off the road (Statutory Off Road Notification/SORN) or it's in the process of being transferred to a new...
All of this is clearly on the .gov site. The MoT and Road Tax use different dates and systems. I suspect Road Tax is because it runs to a Tax Year April thru March. But they want the vehicle to be 40 years for the entire calendar year.