So the timeline we have of the Cucking Saga:
May 31, 2017: Phil confirms to everyone that Leanna is now gone.
June 1: Phil slides a "Hey" into Kat's DM
June/July: Phil and Kat are talking, in a "relationship"
August 16: Kat's BF finds out, confronts Kat and breaks up with her (posts the dumb shit about tacos on facebook)
August to February 2018: Ex-BF seems to have given Kat a 6 month period to GTFO, during same time period she visited Phil minimum 3 times (most prominent being the December visit, leading to the Escort Saga).
February 12 (can't remember the exact date, just remember it was right around Valentine's Day), 2018: The Kat arrives with nothing but a bag of clothes and her computer she sent earlier.
Just my opinion, but based on this new information (for the sake of argument, I'm assuming it's true but I'm skeptical) I think Kat was lying to Phil about her relationship status and he had no idea he was cucking someone. She was looking for a way out, Phil messaged at the right time, but she had nowhere to go in case the ex found out so she had to keep Phil a secret. Unfortunately for Kat, the ex found out and she had to rapidly advance things with Phil, probably sweet talking him and giving him sob stories to make him feel more emotionally attached to her. Might explain why Phil was calling her his soulmate after only knowing her for a couple months, which is really odd for anybody to be doing. Maybe Phil knows the full story by now and he doesn't care (he got what he wanted so he doesn't give a shit how) or Kat is still lying to him.