That was a joke as noticed by many that it hold no meaning. From [hollywoodreporter][1]

> 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
> tone-perfect tweet.
> 
>> "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
 tweeted Wednesday.
> 
> (For the record, Brolin was not in Sabrina the Teenage Witch and
> Deadpool is not in the same universe as Avengers.)

Ryan just played on fan confusion.

  [1]: https://www.hollywoodreporter.com/heat-vision/deadpool-2-ryan-reynolds-is-jokingly-upset-cable-casting-993401