I'm not sure if there is any open source XSLT tool as complete as jq is for JSON. There is xsltproc but IIRC it does not support streaming scenarios (jq has some support for streaming processing)
Though, personally, I prefer JSON. Probably due to superior tools (thanks to its popularity) and less-bloated syntax (it is somewhat easier for me to read raw JSON file than raw XML file).
I do not see license in either repository and it seems that this tool only has 30 day evaluation tier for free. Anyway, using this means that you have dependency on a single vendor and you accept their future pricing changes.
If XML tools aren't open enough for certain needs, then sure, I get it. But it's tragic to see highly-engineered, pro solutions just die out because younger devs don't like the learning challenge or because business owners are cheapskates.
Though, personally, I prefer JSON. Probably due to superior tools (thanks to its popularity) and less-bloated syntax (it is somewhat easier for me to read raw JSON file than raw XML file).