During the Sherriff's Press Conference in announcing their identities, he stated that they had met while hitch-hiking.
Many of us speculated back then as to exactly how he "knew" this or whether it was just speculation on his part. That's where the wiki is getting that information from and I do believe it has a link to the presser in the sources it cites.
If it's "factual" (and not Sheriff's speculation) that they indeed met hitch-hiking, there's got to be some information that we aren't privy to that he is using to back that up (which a bunch of us discussed back then) ... perhaps, once they had been identified, they spoke to those who had reported them missing (JF's ex-wife etc), or friends of one or the other of them, and gleaned info from one of them that they were 'travelling around with this guy/gal I met hitch-hiking' etc. Or, in that Christmas day call to the ex-wife to talk to his daughter she asked when he was going to get his butt down there to visit his daughter and he stated something like, "nah, just met a great girl out hitch-hiking and we're going to do some travelling together, so it will be a while".
Maybe ex-wife mentioned something on her missing report when she reported him missing such as, "Missing individual's last contact on December 25th 1975, when JPF called ex-wife and stated he was travelling with unknown female companion he had met while hitch-hiking".
(And even with the above, I don't take that statement to deem factual that they were BOTH hitch-hiking. If JPF had a car and stopped to pick her up, they'd still qualify as "met while hitch-hiking" or "a Hitch-hiker I met" or "I met her out hitch-hiking". IMO).
All purely speculation, we just don't know.