I've mentioned it in the past two commentaries, but when this comic was first posted, it was the one with fancy blue text above the comic announcing a switch to Mon-Weds-Friday story comic updates for a while. I don't know how long this will last, but I prefer five story comics a week, and it is my intention to return to that when able.
As must as I prefer for comics to speak for themselves, I know people are going to ask me about it, so I'm just gonna go ahead and elaborate on Justin's logic here.
Justin has concluded that Luke's ability doesn't directly tell Luke "these people have marks", but that Luke had sensed enough people with confirmed marks for Luke to think that he could reliably tell who does and doesn't have them. Justin is also assuming Luke would not jump to that conclusion based on finding just one person with a mark, hence the "at least two" conclusion.
I'm not sure if that's elaboration or just rewording, really, but as is so commonly the case these days, I'm a bit tired as I write this (hence the schedule adjustment). In any case, deductive Justin is deductive.