What a critic might consider to be rock music, I might not classify as rock music. I'm frequently perplexed at how critics and music vendors categorise different artists. I'm not particularly clued up about the band that you mention, but I was hearing a lot about them a few months back so I went ahead and gave them a listen. I wasn't impressed.
What criteria is being used to determine who the Number One band is? Record sales? Music file downloads? Concert ticket sales? How many times the artist's name comes up in a web search?
The music industry is completely different than it used to be and many fans - especially old school fans, refuse to adopt a lot of the stupid modern conventions that we're being asked to abide by.
I won't deny that they seem to be popular, their videos have a lot of views and they seem to be a band people are talking about, but that's because so many people don't actually know how to seek out and find new music anymore, they simply listen to what's presented to them, what's most readily available and what everybody else is listening to.
They're not Number One for me or anybody that I know, and I know a lot of people who listen to rock music.