Python to English translation: you'll still get presence information about the invisible person when you log in, just the presence status will be 'unavailable' (aka. offline).
This will have false-positives for people that genuinely have logged off (which gives the same <presence type='unavailable'/> stanza).
Another way to potentially check whether someone's online or not is to send a caps discovery request (ie. ask their client what XMPP extensions it support). If it replies, they're obviously online (haven't tested this, some clients may in fact be shrewd enough to ignore such requests when set to invisible).
Maybe it's just me, but I'm not getting any presence information from invisible users (was signed in as invisible from Gmail from another account) when I log in.
This will have false-positives for people that genuinely have logged off (which gives the same <presence type='unavailable'/> stanza).
Another way to potentially check whether someone's online or not is to send a caps discovery request (ie. ask their client what XMPP extensions it support). If it replies, they're obviously online (haven't tested this, some clients may in fact be shrewd enough to ignore such requests when set to invisible).