Chesnaught is not a Check for Breloom because of those reasons:
1- All Chesnaught's I've been seeing have this move pool:
Seed Bomb / Wood Hammer
Hammer Arm / PuP
Spiky Shield / Leech Seed / Pain Split
Substitute / PuP / Bulk Up
I've never seen a Poison Jab on a Chesnaught.
2- You are only counting the regular Life Orb Technician Breloom (Which is out of date IMO) I'd recommend Sub with Technician or Scarf, Sub + Poison Heal is good too
252 Atk Life Orb Breloom Focus Punch vs. 252 HP / 0 Def Chesnaught: 269-317 (70.78 - 83.42%) -- 68.75% chance to OHKO after Spikes, In My Opinion, in this gen you are seeing Spikes more than Stealth Rock (On Showdown too) because of the following reasons: Klefki and Greninja, which are clearly everywhere.
252 Atk Life Orb Breloom Focus Punch vs. 252 HP / 0 Def Chesnuaght: 269-317 (70.78 - 83.42%) -- guaranteed 2HKO after Stealth Rock, in this case or if you had no hazards at all: Chesnaught breaks sub and next turn is yours to kill him.
3- In some of you calcs, you had been counting Breloom using Mach Punch, now why would breloom use Mach Punch? Being faster than Chesnaught he has: Focus Punch if he had Sub, Power Up Punch twice (if he had sub), Force Palm (not to mention 2 of it if he had sub).
4- A Check is a pokemon that can switch in to Breloom, take entry hazrd damage, take hit, take a hit next turn, and OHKO Breloom with enough health that he can survive any recoil (If any) in ALL of Breloom's sets + Counting the chance of say, not moving due to paralysis (Force Palm), Celebii is the only pokemon who can do that. Counters are pokemon that could defeated Breloom if the pokemon and Breloom are on a 1vs1, Like I said not many chesnaughts carry Poison jab, and if it doesn't Hammer Arm has a 6.25% chance to OHKO, but if breloom survives, it can kill it.
SO chesnaught is not a Check for sure, it could be a counter but that's really counting on that 6.25% chance (Or if you happen to have Poison Jab), if not, Breloom can 2HKO with PuP OR Force Palm.
P.S: This was mentioned: I don't know if breloom learns Play Rough or not, but it's completely useless on him.