Would active duty personnel attend reclass training at a National Guard base? Is this a mistake?

It is happening more often. The Army is going to a One Army School System which ensures all training is the same whether it is NG, USAR or AC. So, no! You should not be concerned.

No reason to be concerned or a mistake. All NG MOS producing schools are TRADOC certified. You will get a quality course.

I currently instruct at an NG schoolhouse and I went through the course myself several years ago. I currently instruct 31B but my brother, also a SSG, teaches 19D. The quality of the training is good and often can be better than AIT because we aren't having to teach you more basic soldiering. The expectation, as a re-class, is that you only need to be taught MOS specific training. I can't speak for all training sites but I think ours does a fine job of training soldiers of any component.