You plan a video conf using their web app, connect at the right time, and show your passport when asked.
As an aside, I login without using their app, as my Android phone does not support Google Play.
Don't know what happens if you don't have a dutch passport though. I guess they are under no obligation to render services to people that are neither citizen nor national.
A bit like when I got married and the French state wanted proof that I wasn't already married before, during the period I had lived in the UK. The UK services wouldn't give me the time of day, since I was neither British nor living there. I ended up getting an official looking note from the Dutch embassy to the UK, stating that "to the best of their knowledge I wasn't married" =)
No problems using similar UK services for EU citizens I know, nor non-EU. Usual bank/address shenanigans at the start, but no issues with government gateway etc.
https://pasteboard.co/2vRyDDgKlHCV.png