Exactly, and they only decided to write those bits in assembly when they identified it as a hot code path AND the assembly outperformed any C code they could come up with.
The times that assembly outperforms a higher level language has reduced as well over time, with compiler and CPU improvements over time.
Which is not in disagreement with my comment/take.
This is largely inaccurate. Video encoders/decoders are typically written in C, with some use of compiler intrinsics or short inline assembly fragments for particularly "hot" functions.