In routing software the bridge surface is mentioned as dirt road both in the Strava route builder as in Komoot.

Bitmac might be the most accurate term for this bridge's surface, but might it not be more pragmatic to just call it asphalt, so that routing software just interprets it as a suitable cycling road surface?

Or should I forward this question towards strava/komoot to update their route builders?

This is no longer Fietsen Verkaemer. It’s still a bicycle shop, but it is now Velo Vainqueur (https://www.velovainqueur.be/)