Maybe it’s just a case of speaking to a different motability customer care as it may of been confused before as the rules do state UK drivers licence only. But in the same sentence it all so implies it’s just further information may be needed.
We will only accept drivers who have a legally valid driving licence. Drivers with a non-UK driving licence will be subject to additional checks.
So maybe worth a try with another handler as additional checks usually just requires extra identification or Proof of address etc (with lots of forms you have a list of valid id, and in absence of 1 or 2 from list A you can provide 3 from list B as an example)
im sure it’s an oversight, it sometimes depends on the person on the end of the phone and which source they read and interpreted it. keep us posted to how He gets on.
Source
https://www.motability.co.uk/contact-and-support/faqs/who-can-i-put-on-the-insurance