I am active on opensource design since several years. Back when we created the logo we had many discussions what would represent design well. Problem is that design often means something rather abstract and it easily leads to boxes and/or arrows to signify "process" or "concepts" etc. If I remember correctly I would have preferred a pencil but, admittingly, it has a less recognizable shape in the flat-shapes-design of today’s icons.

> why they choose a brush to represent the project.

A brush can be a tool for design; graphic design was analog before it was digital. The brush is at a particular orientation, reminiscent of a person looking forward. The human aspect is part of their focus, so the logo fits the dual meaning.

I think it works nicely with this logo https://opensource.org/

Thanks, now I can see why they went in that direction. It makes sense in this context.