That is true if you have something like an ongoing green card petition. However, if it's just an H1B, by the time it's approved and can transfer it, there's not really a "line" anymore.

Though there's pretty hard limitations on what you can transfer with - it has to be the same sector, similar limitations on minimum salary, and requires work on the new employer's part to move the H1B to them (so you can't keep it quiet, and it's another barrier as it's non-zero cost for lawyers etc. to actually do that).