It states that the market decides what? The article repeatedly suggests that what the market decides is which language is "good" or "better," which can only be true if people choose based on quality.
If the market choice is only loosely correlated with quality, then looking at the market tells us nothing about which languages are good (since "good" is a qualitative judgment).
There are only two ways I can see for market penetration to be relevant to the discussion of which languages are better:
1. The market chooses based on quality
2. "Good" is synonymous with "popular"
Otherwise you're measuring one thing and then announcing another. Market position is just popularity, and is only a reliable indicator of the things that determine popularity. If people don't choose based on the quality of a language, then looking at popularity to figure out which languages are good is like looking at the eating habits of the average American and concluding that Big Macs are the most nutritionally balanced food around because they're in the dominant market position.