Aside from their own work, Forbes has contributors whose articles and opinion pieces are not reflective of Forbes itself. What you have on your hands here is an article by a contributor, so the question of reliability concerns only this particular contributor. Not that it matters, because he is relaying stats compiled by GamingSmart, so the real question is how credible that website is.
It always raises eyebrows when a website pretends that the console market consists of only PS and Xbox. But anyway, their methodology is a mixture of numbers by Statcounter, VGChartz and Google Trends; the latter means that number of searches went into the GamingSmart results which isn't a good indicator for actual sales. But this explains how Microsoft's share could rise during years where we know that it has gone down due to the estimates we have on this website here; VGChartz cross-checks their own estimates with data of official sales trackers and tidbits by analysts who have access to such sales data, so they are quite accurate over time.
GamingSmart's methodology is very much questionable, hence why their stats have to be dismissed. Some of their numbers can be close to reality, but they are playing a game of chance where they are bound to get something right every now and then.
I see... thanks for the input.