Yeah, you've got to have a bicycle really sideways at a really high rate of speed to highside from releasing the brake on a locked-up rear wheel.

Unless of course you go from being sideways and locked up on a patch of sand onto a patch of clean pavement or locked up on a wet manhole cover to a patch of damp pavement or a patch of black ice onto not so icy road ...

...well that stuff doesn't even require being locked up in the brake department either, just need your rear wheel to be sliding sideways a bit, then flip!