I'm not sure SBMM is there, stolen from the Netduma forums:
This is exactly what I think too.
Super Sweaty Steve* and Try Hard Trevor* already have the season pass, have the DLC downloaded and will continue to play - so this will feel like SBMM is in full effect.
Meanwhile... Casual Colin* who has not yet turned on his PS4 for the update and Laid Back Larry* hasn't even purchased the season pass are yet to play BO3 since the update to bring the matchmaking back to how it was.
Also, little Timmy No Thumbs* won't see the DLC or season pass until his parents give him some pocket money to buy it.
Until all/most of the existing player base have the DLC, then only the 'hard core' player base will be playing.
*names added in for comical value - I do not endorse or know any of these people.