It depends.

From: https://openbaarvervoerbelgie.be/over-ons/

* For companies publishing real time positions (STIB-MIVB, TEC), live position will be shown. For companies that don't (De Lijn, NMBS/SNBC), we calculate the position based on the schedule (GTFS), corrected with published delays (GTFS-RT).

Every vehicle on the map is labeled with realtime / calculated position.

When you hover over a dot you can see whether it's the real position or whether the position is computed from the timetable with delay applied. The transparent yellow dots are computed, the opaque yellow dots are real position. Same for transparent vs opaque gray and orange dots; the blue dots all appear to be computed (makes sense, the NMBS doesn't provide real-time info about where trains are as far as I know).

For most of them it says "real position" and some show a delay.