Nik version numbers are confusing, see here: https://forum.dxo.com/t/nik-collection-update/10995
A few days ago, I also got the message that an update was available. Updated it with something called “Nik_Plugins_2.3.1_Hotfix”. Nevertheless, my Mac’s information dialog says that I now got version 5.0.2, internally, CFX shows version 5.0.2.27, still the updater has the version number as mentioned above.
This hotfix fixed an issue I had with the Niks NOT showing the image that was to be edited as posted by someone else (add link here if found)
I suppose that 2.3.1 is DxO’s internal version numbering while 5.x is counted from original versions (before google) - nevertheless, numbering is a mess (just my opinion)…