Linux went from 2.59% to 2.89%, that's a 11.6% increase in the number of Linux users.
If it shifted .3% it would have went from 2.59% to 2.5977%.
The article is confusing 'percentage points' with 'percentage'
Another way of looking at it is that the Steam Linux user population went from ~3,418,000 users to ~3,814,000 users. So there are nearly 400,000 new Linux gamers.