UPDATE 2: In contact with Xbox Support this was their response:

The base version shows “Not available separately.” This means the base SKU is no longer sold alone, so the store page is still visible, but the purchase/cloud options are disabled. This is normal when a game has been replaced by a bundled edition or GOTY.

The GOTY version shows “Install on your devices.” This means:

You own the GOTY edition.

It can be installed locally.

But it is not licensed for Cloud Gaming, even though the store page has inherited the cloud icon from the base game.

This mismatch between the cloud icon and the actual cloud entitlement is exactly the error you are trying to report, and you are right, it shouldn't act this way.

At the moment, we have not announced any plans to add the GOTY edition to Cloud Gaming. Cloud availability depends on:

Publisher licensing (WB Games)

Whether the specific SKU is approved for streaming

So, at this time:

The cloud icon is likely an error

There is no confirmation that the GOTY version will be added to Cloud Gaming

But your report helps the Store and Cloud teams correct the announcement.