Am I correct in assuming that Sableye is the only pokemon in the game that can switch in on any attack from Mega Kangaskhan and then win the 1v1 matchup?
Skarmory cant take Seismic Toss twice (which seems to be growing in popularity), and even Terrakion has a chance of getting destroyed before touching Jolly Mega Kangaskhan:
First PuP hit: 252 Atk Mega Kangaskhan Power-Up Punch vs. 0 HP / 4 Def Terrakion: 94-112 (29.1 - 34.6%)
Second PuP hit: +1 Atk Mega Kangaskhan Power-up Punch (divided by two) vs. 0 HP / 4 Def Terrakion: (21.3 - 25.3%)
Sucker Punch: +2 252 Atk Parental Bond Mega Kangaskhan Sucker Punch vs. 0 HP / 4 Def Terrakion: 138-163 (42.7 - 50.4%)
Total damage range: 93.1 - 110.3%
Terrakion isn't remotely safe unless it's at 100%, holding leftovers, and hazards are cleared off the field. The calculator does not accurately determine the damage of PuP, because it does not factor in the second hit being at +1.
I looked into fast fighting types that can take any hit from Mega Kangaskhan on the switch, survive a +2 Sucker Punch and OHKO back. Cobalion and Terrakion are the only two pokemon that really stand a chance but are still unreliable counters. Even though there are plenty of pokemon that can easily revenge kill Mega Kangaskhan, it still has the ability to come in repeatedly and switch out after it wrecks face. Throw it on a team with a couple fighting resists and you're all set.
If, by "win", you mean able to KO Megakahn without switches on either side, then yes I think Sableye is the only one that can do that. As you showed, even Terrakion can't win 100% of the time, even at full HP.
However if you're looking at what can take any move and "win" by crippling Mega Kangaskahn, here's the list of Ghost-types I made earlier (someone else requested I re-post this anyway):
Let's clear up some of the Ghost-type counter confusion. First, here are the calcs for Ghosts that at least stand a chance switching into a Megakahn Crunch, in order from best to worst:
252+ Atk Parental Bond Mega Kangaskhan Crunch vs. 252 HP / 252+ Def Spiritomb: 96-114 (31.5 - 37.5%), Earthquake: (38.8 - 46.3%)
252+ Atk Parental Bond Mega Kangaskhan Crunch vs. 252 HP / 252+ Def Sableye: 121-144 (39.8 - 47.3%), Earthquake: (49.6 - 58.5%)
252+ Atk Parental Bond Mega Kangaskhan Crunch vs. 252 HP / 252+ Def Eviolite Dusclops: 114-135 (40.1 - 47.5%)
252+ Atk Parental Bond Mega Kangaskhan Crunch vs. 252 HP / 252+ Def Gourgeist-Super: 177-210 (47.3 - 56.1%) 21.5% 2HKO after Lefties
252+ Atk Parental Bond Mega Kangaskhan Crunch vs. 252 HP / 252+ Def Cofagrigus: 156-186 (48.7 - 58.1%) Mummy prevents the 2HKO
252+ Atk Parental Bond Mega Kangaskhan Crunch vs. 252 HP / 252+ Def Jellicent: 255-300 (63.1 - 74.2%)
252+ Atk Parental Bond Mega Kangaskhan Crunch vs. 252 HP / 172+ Def Trevenant: 261-309 (69.7 - 82.6%)
252 Atk Parental Bond Mega Kangaskhan Crunch vs. 252 HP / 72 Def Gourgeist-Small: 204-240 (64.9 - 76.4%)
Spiritomb and Dusclops can take anything Kanga can throw (I'd always always pick Spiritomb thanks to its Leftovers and Sucker Punch access). Cofagrigus can also take anything by virtue of Mummy removing Megakahn's ability.
Of course as stated many times, WoW isn't really a "win" since Megakahn can force even Sableye into a 50/50 guessing game... use WoW and potentially be screwed over by a Talonflame/Heatran/Chandelure/etc switch, or go for Recover and eat another hit that outpaces Recover+Lefties?