Spite, or to make the point that they have nukes and the capability to deliver them. An ICBM - or an IRBM for that matter - evokes nuclear fears, which is why the non-nuclear part needed to be clarified. They’ve been making threats about that recently, so I suspect this is Russia’s version of passive-agressive.
Also the range is the only thing that’s different between them, they can both carry nukes. I suspect they did this to make that point. They want Ukraine to be afraid of launching deep attacks.