Are you sure?
When I check Thalys fares on a random date (June 5th), I see fares from Antwerpen to Amsterdam starting at €29 (first train in the morning).
To catch that Thalys in Antwerpen from Brugge, I'd have to pay €36, and also from Gent it would be €36. So the ABS "surcharge" is always the same €7. The total fares only differ when you get routed via Bruxelles-Midi, probably because the base Thalys fare is more expensive from there than from Antwerpen.

Cowboy, I see what you mean; that does seem to be the case on Thalys' site. There is no 'ABS' option though - you put in the names of the stations.

I usually use the Belgian rail site to book; and this gives me a lower fare when I travel from somewhere nearer Antwerp. It's 31,40 to Amsterdam on Thalys via Antwerp then, instead of 36 on the Thalys site.
The Belgian site takes into account the distance traveled to Antwerp, so is cheaper if you start closer to Antwerp.

You find the ABS/EBS option under Aanbiedingen & Tarieven on the top of Thalys' website:
https://www.thalys.com/be/nl/aanbied...ebs-van-thalys

But, as you wrote, it would be nonsense to use that option if you can get a better fare on SNCB/b-europe

Well other day looked at Thalys for one month out and there were many 36-39 euros available for Bruges-Amsterdam. That said the two experts above find out otherwise and I defer to them - Paris may be different. I'll stop talking about discounts on Thalys that are hard to get. Thanks.

Yes, and then try to book those. First of all, they're never at the departure time you need. Secondly, it has happened to me more than a few times, that the cheap rates "disappeared" while I was in the booking process, because of the run on cheap rates as soon as they're there.

Some years ago, I easily booked 39 euro tickets Par-Ams for friends - I think now Thalys trains are so full that they don't need to discount tickets - cheapskates can take the much slower and less comfy cheap regular alternative train Par-Brussels then ICs from there.