> The risk with printable tickets is that a seller could sell it to multiple people, who all print it out, but then only the first person who uses it can get in?

Note that the portion of that you're quoting that you didn't quote is "If you bought the ticket off the event’s official ticketing agency (not a sketchy reseller)"

I.e., we're specifically talking about someone holding a ticket that they purchased from Ticketmaster. If there are multiple copies floating about, presumably at some point the artist (/the actual event) is going to be unhappy that Ticketmaster is screwing their fans/attendees over.