what she means:
people have this horrible misconception that margaery genuinely set out on the path of 'righteousness' and trusted the high septon, which is what led to her downfall when in fact it was all an act she put up in order to free loras from his misery and have him alive even if it meant he had a scar on his forehead and even if it meant that she'd have to PRETEND devote herself to the faith, we all knew her final and only loyalty lied with her family which can be proved by the fact that she sacrificed herself for her grandmother while communicating to her effectively (by giving her a paper with their sigil, the rose) that she's still a tyrell and always has been one. the fact that people consider her 'keeping her faith' in the high septon as a mistake and her ultimate defeat are righteous idiots because margaery never lost and would've been the only person to make it out of the sept if it hadn't been for those damn guards.
Before he married her, Renly was scheming to get Robert to replace Cersei with Margaery.
Mace and Loras went along with this plan.
None of this actually benefits Renly or the Tyrells unless Cersei’s children could be disinherited as well.
The only way that Renly could promise that to Mace and Loras is if Renly knew about the adultery/incest, which would make Joffrey et al. bastards born of incest.
But, and this is key, the moment Robert dies without having divorced Cersei and disinherited her kids, Renly can’t admit any of this because it makes Stannis the indisputable rightful heir to the Iron Throne.