How can they say the mileage will be reduced to whatever other leases are. You can have any mileage you want with a leased car. When taking out a lease you are asked what mileage you want. Many businesses lease cars with unlimited mileages. Salary sacrifice leases you have to state what mileage you want, the less miles the cheaper the lease. That part is nonsense.
What they are effectively saying is that 20,000 miles Pa is not in line with standard lease deals, most of which are defaulted to an annual mileage of between 8,000 and 12,000 miles. However, as you say, you can have a lease at whatever mileage you want. Therefore, my assumption is that Motability will change the default mileage on a lease to a lower figure (say 10,000 miles Pa) but people will be able to do higher mileage if they are prepared to pay more.
I’m always amazed at how low the mileages are on most Motability cars (which, IMO, makes them not terribly cost effective), so I suspect that this will only affect the minority of scheme users.