I just read the Dakka thread, and I end up siding with Quannar, or whatever his name was.
You read it from top to bottom in the data sheet. Any model can replace ALL weapons (which is the powerfist and stormbolter at the time of taking the option) with TH/SS.
Then you get to the next option, which says you can take any heavy weapon. Ok, you head to that spot, you read... AC? No, you've already replaced your bolter. Plasma? Nope, already replaced bolter. CML? Yes, you don't need to replace your stormbolter.
You're simply going in sequential order. There's already a precedent to go with it; take a look at Farsight, for instance. The infamous 'Eight'. nothing can join a monstrous creature, so no one can join TO O'Vesa (the riptide). But any IC can join a group, so O'Vesa can JOIN, just not BE JOINED.
it's all a timing issue. I would absolutely still allow it, because RAW, every rule is satisfied in the order it was presented to the player.