You are right. If you want grass to grow in a bare spot right away, mulching is not a good idea. Mulch works well around trees, ornamental shrubs, and edibles. Mulch will smother out grass.
However, if you want the grass to grow in the bare spot
next season, mulch is a good idea, especially if you put a little compost under the mulch. The theory is that the microbes in the soil work best when they are kept at an even temperature and moisture level. Mulch protects the surface of the soil from the sun's rays and from the wind. Both of those will dry out the soil and microbes and slow down the microbial recovery underground.
Does that make sense?
If you want to encourage growth immediately, use a little compost, not nearly enough to cause a protective mulching effect. One third of an inch is all you need. Then dampen that to wash the microbes from the compost into the soil. It takes about 3 weeks to see any difference, which, when you are waiting for grass to grow, seems like eternity.