?

Likely better discussion for the discord but I can answer, as it took me a while to figure it out too.

I reached out to another runner on YouTube to eventually discover the secret!

Basically you have to do a quick save, restart the game then continue with a quick load. When you do this, something is not set correctly and it gives you further grenade distance. This is why the WR run currently quits at the start of the Bocage level and reloads - this is how you do the grenade trick on that level.

Cheating? Absolutely not - its a quirk of the game / engine and things like that are the point of any% runs IMO.