As I said in the other thread, the plugin does not make the assumption that there is only 1 object selected. It in fact works a loop to iterate all objects in the selection and assign them to use the chosen tag. (If there is only 1 object in the selection then the loop runs once.)
Regarding menu styling. We cannot embolden menu items, but we can disable and grayout items.
We can “mark” items with a checkmark, which is usually done to indicate a toggle set in the affirmative position (true, on, etc.)
It would not make much sense to check more than 1 tag if the selection had multiple items using different tags.
Currently the tag setting loop uses a conditional expression that only makes a reassignment if the object’s tag is not what the user chose via the submenu. So even though the loop runs when you chose the same tag an object is using, no undo operation is created as no change to the model occurs.