Largest petition on the site ever is saying something, though. Since other petitions that have attracted less volume include far less controversial topics such as increased funding for schools, it is still notable. In essence the mistake is to assume that only those signing the petition believe in it, rather than, what is slightly more likely, many people agreeing with the petition but viewing it as a waste of time.
And in any case, is the implication that the petition can only be taken seriously the instant it exceeds the 2016 referendum result? That seems unlikely. A more reasonable comparison might be to the current opposing petition, ie one calling to leave without a Deal this Friday, which is not yet at half a million votes. I would guess that this doesn't represent the total number of leave voters either.