I don't believe that info at all. There's only two screws holding the modem together so you can check the filter caps if you want.
first off, that's not the modem you have. You have a 28.8 sporters so it's very old, and not the latest 33.6 they may be selling now which they may have given you specs for.
none of the 28.8 or first 33.6 sportsters used 20 volt adapters. Only couriers did.
They still make couriers but switched from AC adapters to DC switching power supplies and changed the power connector several times.