I wouldn't call IDX core because it doesn't solve the problem the article opens with. It says that modern app stacks are complex. IDX doesn't solve that at all. There's the same amount of moving parts behind the curtain, it's just that stitching them together is made easier.
The motivation for the service is unclear, but in itself it can't produce serious revenue so I guess it's an on-ramp to Google Cloud hosting. And perhaps another "free" service to train AI on.
We can certainly agree that to "navigate an endless sea of complexity" in getting an app "from zero to production" is no fun. Firebase is in the business of making this easier, and IDX is probably, as you might imply, an on-ramp to use Firebase. I don't see what is wrong with that.
I also fail to see why IDX would be free. I guess it will run in the Google Cloud, and provide their developers with some hard earned income.
All in all I'm still positive in hoping that there will be a real solution for a real problem :)
My neutral stance is that this is a potentially marginally useful tool. There's nothing fundamentally new about it. Github has an online IDE, as does Amazon, as does everybody. In particular at Github it shouldn't be that difficult to connect it to deployment actions, after which you have the same thing, yet with more flexibility.
As said earlier, it doesn't reduce the complexity of the tech stack nor will it maintain itself. When you create a React/Vue app this way, your shit continues to constantly get out of date. And you'll need to continue to coordinate dev setups within a team.