I've seen a college professor cite wikipedia in support of a false claim. On investigation, the text in wikipedia was cited to an earlier blog post by that same professor.
I wasn't convinced.
1- citing wikipedia (or any tertiary source) is valid, the problem is just when the source is not cited. And also it's against wikipedia policy, but you are free to cite it elsewhere.
2- citing the tertiary source and citing the secondary source are distinct and valid. There is no "rule", in wikipedia or otherwise, that says you need to cite the underlying source. In fact citation chains can become quite deep, it would be very impractical. An example would be, you could cite the gospels when jesus talks with the devil. If we had it your way then you wouldn't be able to cite an apostle, you would have to attribute the quote to jesus, and furthermore if jesus quoted the old testament you would have to cite that? If you think the bible is an exception, consider case law, if you were to cite an attorney's defense and the attorney cited some cases, would you have to cite the original cases? If so, then which? There might be multiple, it's not just a citation chain but a graph.
In this specific example your professor was not just quoting himself, but his work is now part of wikipedia and importantly was not contested or was not successfully contested. Similarly to how a trademark works, you claim you own the trademark, and if a year or so no one contends it, you have a stronger case that it's yours.