That number could refer to a public transport zone which is used for certain subscription types that are valid within one or more zones. Here is a tool that shows all zones (first select Arriva, then Limburg). Aachen would be 6646. The bus stop in your example apparently lies in the Vaalser Quartier zone. So yes, the numbers are still valid.
Wether the tag name zone is appropriate, I can’t say, but at least it doesn’t look like an ideal situation to me.
Since TEC is the name of a bus company, I wouldn’t use that scheme (e.g. zone:Arriva), because the Dutch zones are not specific to one particular bus operator.
I noticed that bus stops in The Netherlands seem to have the zone number in a ref:IFOPT tag as well (e.g NL:Q:xxxxyyyy, where xxxx appears to be the zone number), but I don’t know how that would work for foreign stops frequented by Dutch operators.
Maybe someone in this thread could shed some light on this issue.