Check the related Microsoft blog post for details on "Protecting the Integrated Search Experience in Windows 10;" later in the BBC article: "Statistics gathered by Statcounter suggest that globally more than 90% of searches are carried out via Google. By contrast, Bing accounts for about 3% of searches."
"Windows users will still be able to install other web browsers and use other search engines outside Cortana, said Ryan Gavin, Microsoft's head of search and Cortana, in the blogpost.Microsoft stops Google being used for Cortana searches - BBC News
However, he said, Microsoft was adding extras to Edge and Bing that meant it made sense to tie these programs to Cortana instead of other search engines and browsers.
Anything else would be a "compromised experience that is less reliable and predictable", he said."