Doesn't that sound low -20 dbs
Yes. And yes, it's lower than almost any audio CD, mp3, podcast, or on-air broadcast...
any other questions?
We got the -20 spec in production recording for practical reasons: to allow enough headroom for sudden peaks in a performance, on early equipment where even a couple of milliseconds attempting to record above fullscale would give you horrible clicks.
We got the -20 spec for post because Sony designed their Digibeta decks so that -20 dBFS on the tape would generate +4 dBu on the analog output... the same level that 0 VU would in an analog Beta deck. This was a day when facilities were just starting to convert, and digital decks were often wired to analog consoles or patchbays.
Ditto -10 dBFS absolute top for peaks. This was considered the same practical amount of headroom above "zero level" -- i.e., -20 dBFS -- that you'd get from a good analog deck. (Headroom in analog is a slippery topic because there's no definite point where distortion becomes intolerable, so long as you don't hit the power supply rails... but that's a topic for some other thread.)
I deliberately wrote "top for peaks" rather than "limit". There's nothing in the spec that says you have to use a limiter, and certainly nothing saying you have to slam that limiter and destroy dynamics. The limiter is just a convenient tool to assure safety in an otherwise properly done mix.