I tested the code on the bp-default theme (only wordpress and buddypress activated). It works on normal pages, but not on the buddypress pages (members/activity etc). Hence, it behaves the same as on the pagelines theme.
I tried different priority settings for the filter in the plugin, but without any results. Any other suggestions I could try?
Naturally, I posted this question on their forum. They answered that it is probably a buddypress problem, because they are using a wordpress filter. The plugin does add body classes (for browsers and devices) on all other pages (non buddypress pages).
Why should this not be working on buddypress pages? Yes. I have little knowledge of buddypress, that is why I am here.