I agree that Giyuu wouldn't be USELESS but to say that he would go toe to toe with him until help arrived, I can't really picture that. And even IF the help does arrive, it's not like they would help, let's say the 2 pillars that beat upper moon 1 come and help, would that help at this point? NO. Like I just said in my previous post, the only way they will be able to even have a lick of a chance of beating Muzan is to bring him outside, which means the defeat of UM4 comes first before anything else, which is why if you're a Giyuu fan you may want to have Mitsuri and Iguro get this fight done quickly, or else it'll be not only a quick death Giyuu but everyone else there.
Also, I think we need to hold back on this Muzan is invincible thing. I mean yes he killed many demon slayers in such little time, but come on now those guys were babies compared to the pillars and you could argue the top 3 UM could have all done the same thing. I'm not saying he NOT as powerful as the the rest of the upper moons, I'm just saying lets not all crown him all of a sudden for taking out trash tier demon hunters. Another thing is the demon antidote.
I'm gonna say it DID work, but not fully yet. I think the drug is inside Muzan but has yet to take its effect. Almost a bit like how Kocho poison didn't come into play until WAY later. I'm going to say after UM4 goes down and the fortress crumbles, The antidote will now start to take its effect on Muzan, and he will slowly and slowly begin to turn back into a human.
.............
Or you know, it really just didn't work because Muzan daddy is that much of a boss.