Tim well, actually, since Apple Pay launched (from the big banks with Maestro only) in The Netherlands, the Mastercard infrastructure needed to support Maestro online is now operative in the Netherlands this in turn links to pre-existing infrastructure for Maestro online from countries such as Spain, Italy, Portugal, Belgium and the United Kingdom where Maestro was historically already used online (by some bank), which means any bunq user with a Maestro card from any of the countries bunq offers Apple Pay in, should work online (on it's own and also through Apple Pay or Google Pay if supported specifically for that last one; I'm not sure), though this is not guaranteed, and maybe not yet at every webshop that support Maestro.
To sum up, it's a bit tricky currently.
So it would be complex for bunq to offer this and not able to guarantee that all foreign Maestro cards from other banks would work.