"Non-regrettable" attrition means either that the person was fired or laid off. Since the post says Automattic didn't do layoffs, that means all of that attrition took the form of being fired because of performance problems.
If 7% of the company exited the company in a year because they were fired -- which seems quite high to me -- then even if you don't call it a layoff, it has many of the same characteristics as a layoff.
In a classic layoff situation, you've over-hired, expecting future growth, and then the winds change, and you need to reduce the workforce to match the declining growth.
In a case where you're firing 7% of the company because of performance problems, you haven't over-hired, but you have certainly poorly hired!
Either way, now the remaining workforce has to adjust and "do more with less," and I'm going to guess that most of them have a reasonable fear that they might be next on the chopping block!