Yes, there is, it's just not very intuitive because although it is a configuration option, it's not related to mobile devices itself.
Assuming you have a Wikipedia Account and that you're already logged in, go to your Preferences (top-right links) and under the Appearance tab select MinervaNeue among the available options for Skin and hit Save down below.
This will activate (almost) the same theme used by default while on mobile. Almost because you can notice a few things different if you were reading/editing in a mobile device. A few things I've noticed from a quick look that's different in the mobile version:
- The column with assorted information most of the Articles have to the right is a little bit further down to not share space with what I assume are the Articles' headlines (first paragraph)
- Right-aligned images don't have that extra bit of top-margin
- Disambiguation message is not text-centred nor in italic
- There's no "purple box" at the end of the Article, below the usual References
- There aren't many links in the site's footer
I didn't go too much deeper but as you can see, it's minimal. If you really want the full mobile experience on Desktop, you may use a browser extension that allows you to change your User-Agent like in the answer I posted in the link you mentioned.