Yes, the AI tag is the AI Alignment tag.
In addition to most discussions of AI being about Alignment, "AI" is also shorter and allows us to fit the AI Alignment tag more easily into the frontpage-tag filters and on post pages.
There is no tag that encompasses all of AI alignment and nothing else.
I think the reason you gave is basically correct - when I look at the 15 posts with the highest relevance score on the AI tag, about 12 of them are about alignment.
On the other hand, when a tag doesn't exist it may just be because no one ever felt like making it.
Done.
Why is there no tag for AI alignment or AI safety? Is some other term used?
I see a tag page for AI, which lists tons of really specific sub-tags. I notice that a lot of the sub-tags are about alignment topics; maybe the "AI" tag is effectively the AI alignment tag, because LW cares about that so much that we effectively never talk about AI not in the context of safety?