On top of the scripts page one can read this:
Even though your browser is not in the list,
you can use the script if you use one account for that script.
I guess that is what I experience with my tele2.nl script. I have a zonnet.nl and a tele2.nl account at Tele2. I have them both configured in XN, but with Chrome one is always greyed out. The greyed-out account gets activated when I click it in the XN-icon popup menu and the other account gets greyed out. I reckon this behavior is the same for more than 2 accounts.