IIRC this timestamp is updated if e.g. the user makes a post, sends merits, etc, which you can also see on Bitcointalk itself if you look at user's last posts or recent merits. If the user is just browsing the forum then BPIP won't be able to "snitch".
But does that “etc.” count, for example, if someone adds someone to his trust list?
Also, I was thinking that, for example, an unregistered user cannot see if someone has sent merits by entering their profile. Although it would take a few minutes to register and see it.
In the end, I see that this change that they made for privacy reasons, which caused many of us to activate the option, is not really a big change. He did it with good intentions, though.