As far as I can remember, the station code—which is still used internally to determine whether displays inside the station will be put with French first (odd numbers) or Dutch first (even numbers)—is still 27, and still matches this part of stops.txt GTFS.

Middle two digits of the platform code:
8271,,"SAINTE-CATHERINE",,50.852347,4.347749,,,0,5
8272,,"SAINTE-CATHERINE",,50.851900,4.348012,,,0,5

And first 3 digits of each exit:
0270105,,"1 - Quai au Bois à Brûler",,50.851934,4.348232,,,2,5
0270305,,"1 - Quai aux Briques",,50.851853,4.347863,,,2,5
0270405,,"1 - Quai aux Briques",,50.852581,4.347365,,,2,5
0270505,,"1 - Quai aux Briques",,50.853165,4.346953,,,2,5
0270605,,"1 - Quai au Bois à Brûler",,50.852949,4.347536,,,2,5

You have a valid point that GTFS data also contains additional entries for station themselves.

5,,"SAINTE-CATHERINE",,50.851900,4.348012,,,1,

It follows another sequence, where stations were inserted into the GTFS from west to east starting from Gare de l'Ouest/Weststation.

Honestly, I don’t know what to do with that situation, because both numbers are correct…