@NT I had to research this a little, and what I found is that the base_table attribute only applies to the table name field associated with the document id field, but not the document_id field itself.

If you are showing both on a form, the table name field should be limited by base_table, and document id limited by table name.  But, if you search on the document id field with the magnifying glass, however, a GlideModal form of the document_selector ui page is presented.  That ui page makes no mention of the base_table attribute, and, as you have found, you can select any table.

I don't see an easy way to update that ui page, and even if I did, I would recommend against it, as that is an out of the box ui page that may be updated in a future version of ServiceNow.

There may be an attribute you can put on the document id field to limit it, but I didn't see one either by looking on the dictionary record for a document id field, or by looking through the document_selector ui page.

If you really need this functionality, I recommend reaching out to ServiceNow's HI team to see what they have to say, as I don't see a way around the current implementation without risking issues during future upgrades.

Good luck!