I would be tempted to agree with you, Dan. Although it is the first direct bus serving Heathrow Terminal 4 in the time period specified, there are quite a few non-direct services available which would have been faster, and therefore I would argue that waiting for almost two hours for a direct coach would not be realistic.
Having said that, we're in Heathrow Terminal 4 so I don't see why we shouldn't board a Piccadilly line service (leaving at 20:43), and ride the train to Covent Garden, where we arrive at 21:44 on Wednesday evening. As we leave the station, we are reminded that it's an exit only station.
Above post removed - out of sequence.