Upvote:3
One purpose of the check-in is to get a document or code allowing you to board the plane. Even if it is paperless, it's still a distinct step for most airlines. It is often possible to do it online without going to a desk but that's really up to the airline.
Note that Thalys is a train with mandatory seat reservation, it's not permitted to board a train if you do not have a specific seat assigned (in practice this is checked a lot more systematically in Paris than in Antwerpen but that's the principle). Only frequent travelers with Thalys Club membership or some premium ticket holders are allowed to board the previous or next train (βframingβ trains) and even then they are supposed to approach a train guard to get a seat. So your boarding pass will only cover one specific Thalys train.
On the other hand, if you did get your boarding pass for the flight, it may still be possible to use an IC direct and just bypass the Thalys leg (see Can I skip my train leg booked with a KLM flight?). Or, as suggested in a comment, to ask the desk for a boarding pass covering an earlier train. But whatever you do, you have to comply with the airline's process to get a boarding pass, just ignoring check-in and showing up at the airport is asking for trouble.