They’re not the same statement. Both are correct. But as always, the nuance is critical.

All declarative code eventually runs something imperative.

And anything non-theoretical is going to have side effects somewhere.

Painting to a screen is a side effect.

Reading a file is a side effect.

Software gets messy when it intersects reality.

But, we can do quite a lot by staying in a theoretical realm. You can build a whole UI in the theoretical realm, where no side effects exist.

When you actually need to run it though, you drop out the the theoretical and into reality. Now there’s side effect’s everywhere.

So you contain them. Encapsulate those side effects into composable blocks. Test them extensively. Offload that work to someone who’s just focussed on managing them.

Updating the DOM is a side effect. Making a network request is a side effect.

Contain them, test them, and then you can stop thinking about them.

If someone else has done all the work to contain side effects, you can build a UI without them.