It's not better out of the box for text to image, this was known for quite some time. However, as soon as they release the weights (in a month as they promise), it will benefit from the tooling available for SD, without being limited to text to image.
It's also a foundational model, not a finished product, and MJ will possibly use it, like they did in v4 with SD 1.5.
Midjourney only used a combination of SD and their own stuff with the --beta and --test/testp model which came between V3 and V4, other versions have no connection to SD.
I might be misremembering, but didn't they announce in their twitter that they were using SD somehow for MJ v4? Later deleting this with a bunch of other tweets.
It's also a foundational model, not a finished product, and MJ will possibly use it, like they did in v4 with SD 1.5.