Does the brush panel stop dust getting into the rack?

It helps, but it does not seal the opening.

Compared with leaving an empty 1U aperture, the bristles close around a small group of cables and reduce the clear path through which dust and unwanted air can travel.

That is useful, particularly where only a few leads need to pass through.

It remains a brush curtain rather than a gasket, however, so describing it as dustproof would give a modest piece of rack hardware rather more authority than it possesses.