The whole Kylie thing is interesting because the whole Christmas Number One thing has changed so much over the years.
For a long time it was fought hard over, with releases in the 80s and 90s doing battle on sales. Then into the 00s, along came Pop Idol and X Factor, with the winner basically guaranteed to get the Christmas Number One because they were such big shows. Then after those shows died down, there was a period of novelty records which triumphed, before streaming basically killed it all, and it became a Mariah Christmas every Christmas. Wham eventually usurped Mariah in the UK, but this year there is actually a new song at number one!
But XMAS by Kylie is Amazon Music backed. They've put it on all their playlists and pushed it hard. It's got radio airplay, and there were lots of physical versions to buy - many through Amazon. And this isn't a new Amazon thing, they got #3 last year with Tom Grennan while Sam Ryder made #2 the year before.
The UK chart rules have something called "ACR" or Accelerated Chart Ratio that means new music requires a smaller number of streams to be the equivalent of a sale. Billboard has something similar. Still, it actually makes a change for someone other than Wham or Mariah to be number one, even if it's all meaningless!
Adam