Pure sprinters are the soccer players of cycling: they are lazy, they train less kilometers and they don't train that much in the mountains or on the TT bike. Why would they? They have that one weapon (the sprint) that gives them the wins. The whole team will ride for them and even push them up the hills. So many pure sprinters don't show any incentive to do well in TT (why would they? They focus on stages, green jersey, Paris-Tours, Scheldeprijs and their sponsors are happy). Thinking about McEwen, Ewan, Bennet, Blijlevens Cipollini, Petacchi, Cavendish, Kittel, Steels.
TT guys on the other hand are a mix of GC contenders and specialists in just TT. Why those TT specialists don't ever try to sprint: I don't know. Some are loners and ride TT first because they don't like to ride in the peloton or are bad bike handlers (e.g. Tony Martin). Others are only build for endurance (and if they are light enough, they end up trying for GC) . Anyhow, WvA isn't anywhere near being the established alpha male TT. Currently, Rohan Dennis is still the reference, and you have Ganna, Dumoulin, Roglic, Evenepoel and Cavagna is on the rise.
Cancellara, Jalabert are 2 WC TT that have a good sprint. Alaphilippe won the Tour TT last year and has won races sprinting in smaller groups. But riders like Cancellara or Alaphilippe don't want to do mass sprints or rely too much on their sprint, they have capabilities to arrive in smaller groups or solo and will just try to do that, and don't risk crashing in mass sprints for whatever reason. Boonen, Kittel (both of them winning TT in their young years), Cipollini and even Sagan can do very well in a short TT, especially if that gives them a chance on a leader's jersey in a grand (or small) tour. So if they have a reward coming, they can TT. If they don't have any incentive, they see any TT as an obligation / day to rest as much as possible.
WvA is first of all a CX guy with a big engine, trained on doing 1-hour FTP efforts. So his coaches reckoned he could to well in a TT. And CX is explosive so he mixed in sprints on the road from a young age. His best sprints are straight and long. He doesn't have a Kittel/Greipel power sprint peaking around 1600-1700 watts. He does maybe 1400-1500 watts max. effort but can hold that 3-5 seconds longer. He has won his sprints by positioning more than anything else. He has the engine to position himself and can still do his maximum sprint effort. Not many sprinters can do that (they run out of gas if they have to position themselves): Sagan is fairly good at that but mostly because he fearlessly diving into gaps and going shoulder to shoulder, WvA is more reluctant to fight for position.
Matthieu VdPoel is another rider that is probably as good in (short) TT as WvA, but he hasn't done any TT recently. He also has a sprint that's just a bit faster and more powerful than WvA (higher peak watts). But it seems VdPoel's climbing pedigree is lower (and for sure less trained) than WvA. VdPoel hasn't done any long training camps with GC candidates like WvA has, and he seems to be just that bit more explosive and thus better at sprints and less suited for the mountains.
A long story short: WvA is maybe 4-5th in TT, 4-5th in sprints and around 20th in climbing in the current peloton. Does that make his performances exceptional?