Yes, as usual the Reg knows WTF they are talking about
TheReg is probably right.
Here are the Google trends on Wordpress vs Joomla vs Drupal.
The claim that Drupal powers more than 1% of the web comes from Dries Buytaert, leader of the Drupal project. He started a project to build a crawler to categorise sites, and Marc Seeger finished it. Marc wrote his thesis about the crawler. In Dries's April 2010 Drupalcon speech, he revealed that a crawl of the top million sites showed Drupal at 1%. His slides are online. It doesn't say so on the slides, but he also spoke about Joomla and Wordpress stats. I was in the audience, and I remember WP being at about 8%, and Joomla being at about 3. I'm fairly sure about WP, I'm less certain about Joomla. However, Drupal at that stage was definitely behind Joomla.
This also feels right - Drupal is typically used for more complex sites than Joomla. Naturally there will be more less complex sites, so we can expect that there would be more Joomla sites.
The Google trends graph shows that Joomla is dropping in the search rankings, but it's still well above Drupal. Of course this isn't necessarily a reflection of the use of the different CMSs, but I can't think of any sensible reason that it couldn't be.
With regard to whether Wordpress is a CMS. The majority of my business is built around Drupal and I'm heavily involved in the Drupal project. However, one of my clients uses WP very successfully to run their online industry specific newspaper. It's not a heavyweight CMS, but it's definitely improving all the time, and the latest version supports structured data. Remember that it wasn't Habitat that bought Ikea; it's the job of us Drupal developers to make Drupal easier and nicer to use than Wordpress. Drupal 7 is a good step in that direction. The user interface is better, but we still have a way to go to meet Wordpress. On the other hand, the structured data handling in Drupal 7 is in another league compared to Wordpress.