The post does not imply the 5090 is needed, that is just a common reference point.

A single six year old RTX 3090 works great: https://www.reddit.com/r/LocalLLaMA/comments/1vkm42m/muse_gl...

I fully expect Meta will release other, smaller Muse models in the near future too.

The 5090 is also supposed to be a $2000 GPU, not a $5000 one. The entire market is utterly distorted right now, which will impact cloud inference more and more over time too. They are not immune to the absurdly high RAM prices, so their prices will have to go up over time too until the RAM supply chain goes back to normal.