It depends. In summer, it's definitely bright enough with some indirect light. But in winter, pretty darn dark.
Thanks for the tip with the light meter. I already measured the PPFD with my phone using an app and a sheet of paper in my grow tent. It actually worked extremely reliably! It matched exactly the specs of the lamp very accurately.
I'll measure the brightness on different times of the day and weather and then report you back in a week or so if the spots are able to support life :D