That was a joke, as noticed by so many. From hollywoodreporter
Josh Brolin's casting as Cable in Fox's Deadpool 2 has been greeted with both joy and confusion online. After all, doesn't Brolin already play Thanos in the Marvel Cinematic Universe? Is that even allowed? (The answer to both: Yes!)
Deadpool star Ryan Reynolds played up those fan anxieties with a
"The f—, Fox! You can't play 2 characters in the same universe!! Josh
Brolin was in Sicario and I was in Sabrina The Teenage Witch," he
(For the record, Brolin was not in Sabrina the Teenage Witch and
Deadpool is not in the same universe as Avengers.)
Ryan just played with the fan confusion.