The bi(n)g assumption the article makes is:
Firefox switches to Bing from Google and still continues to be equally popular in the medium/long run.
There are multiple possible scenarios -
Post such a shift FFs market share erodes quickly and Microsoft basically bought a Lemon. Chrome becomes even more powerful.
Post such a Shift FFs market share is simlar to that it enjoys today - Microsoft bought some traffic but a a large majority of FF users switch the search engine back to Google. Microsoft bought a sort of Lemon
Post a shift - FF market share stable/increases and lots of people do not change default search - Microsoft bought a winner and Article makes sense.
I do not see #3 as the most likely outcome.