The apt source code has no mention of SOCKS whatsoever:
/tmp/apt-126.96.36.199ubuntu1$ grep -ir socks .
It's also possible to have extension plugins for apt, such as
apt-transport-https. These add files to
/usr/lib/apt/methods/. So I used
apt-file find to look for any other packages that provide files into this directory. This also did not reveal anything SOCKS related.
It's impossible to prove a negative, but I'd say that it's safe to say that apt itself has no SOCKS support right now. It looks like tsocks is the only real way forward.
I'm confused as to how others have concluded that
Acquire::socks exists, but I can find no evidence of it. If I've missed something then I guess this answer is completely wrong (and I'll delete it).