KL2661 seems to be an KLM codeshare on the 7:13AM Thalys train Amsterdam-Paris, but only on the Schiphol-Antwerp segment. After Antwerp the train continues to Brussels-South (ZYR) and to Paris.

KLM have airplanes to Brussels, they don't codeshare on trains for Brussels. If they really mark Antwerp-Berchem station as "Brussels", it's definitely bizarre, Antwerp is a completely, proudly separate city, 50km north of Brussels, with its own Antwerp International Airport ; the station is actually labeled as "Antwerpen-Berchem", in dutch.
If your ticket is marked as you wrote, it might be valid until Brussels proper, given that the train actually stops in Brussels. If not, a railway ticket from Antwerp-Berchem to Brussels (any station) would cost you 6.10€ (you'd need to change to a normal train, no problem, but you'd have to buy a ticket and need to take your luggage down a short flight of stairs, escalators are only for going up). It would be no more of a hassle than flying to Brussels Airport and taking the train from there. It also depends which part of Brussels you want to go to ; all regular trains stop at 2 other stations before Brussels South. You could even ask to include one-day use of public transportation in Brussels in your rail ticket (costs a bit extra, saves hassle).