Just came across this and was also puzzled at what seems to me to be inconsistent UI. When creating a topic you can select tags and create them if they don’t exist. In this instance you can select tags but have no option to create them. As a user I wasted time trying to figure out if I was doing something wrong.
The UI trains the user to interact in a certain way, and this kinda breaks that contract. Seems weird not to support it.
And how do we usually create tags? By creating topics. And what is this operation? Creating a topic.
It’s not even clear to me how to make a tag without creating a topic. Now if you’d said
Split, then tag (and create)
I wouldn’t quibble, though I would disagree.