I'm sceptical. The Belgian deaths the paper refers to are being misrepresented. Belgium is overreporting deaths, compared to many other countries' underreporting. Drawing any conclusions from those numbers does not shine a positive light on the quality of the paper.
Only 0.5% of Belgian deaths are under 45y/o. About 80% of the reported deaths are in people over 75. If the MMR vaccine offers protection, and people weren't vaccinated until 1995, I'd expect to see more cases in the 30-45 age bracket. But its virtually empty.
But then there is a paper from 2014 [1] where the authors were looking into a a SARS-CoV (not SARS-CoV-2!) vaccine based on a measles/rubella vaccine, so maybe there is something to it.
That being said, the lack of updates about this probably isn't a good sign.
[1] https://doi.org/10.1016/j.virol.2014.01.002
