While a literal reading of the MIT license refers to "software", many datasets have been released under it.

In particular, if someone releases something that is only a dataset along with an MIT license file, the most reasonable interpretation is that the rights holder intended to release the data under the terms of that license.

I looked for copyright cases involving this specific distinction, whether "data" versus "software" makes a legal difference, but didn’t find anything.

So the question remains open (for you, for me it's pretty clear the dataset is released under MIT).

You might want to sue and find out. It sounds like an interesting experiment.