That would appear to be true Jojoe, this is what Motability say….
Dartford Crossing
A282 Trunk road Dartford, Essex
Disabled motorists may cross without charge if they are travelling in a vehicle which is exempt from Vehicle Excise Duty (VED). You do not need to register or present any documentation as your number plate will be picked up by automatic number plate recognition (ANPR) cameras. Please be aware that motorists who receive Standard Rate PIP and therefore are eligible for a 50% reduction in road tax are not exempt from any charges.
If you have a Motability Scheme vehicle and receive a penalty charge for not paying the Toll please contact the Motability Scheme as it is possible that your vehicle details were not recorded properly by the dealer.
For more information visit www.gov.uk/dartford-crossing-fees-exemptions-penalties