You’ve been given bad advice.
I spoke to Motability over the phone just yesterday about tinted windows. You CAN add tinted windows at the point of ordering if it’s a factory option. I was actually told that Motability do not even see the options you choose and pay for yourself.
After handover you CANNOT add tinted glass without a valid medical reason and Drs letter and approval from Motability BUT you CAN have tinted FILM applied but you will need to ring Motability to have them add it to your agreement.