Which version of Debian are Ubuntu LTS releases based on?

This answer says:

The base of the operating system, Debian, comes in three versions: Stable, Testing and Unstable. Normally, Ubuntu is based on Testing; the LTS releases are based on Stable.

That cannot be true then for 12.04 because Debian Stable (Squeeze) is almost going to be old-stable, and it's obvious that the kernel versions in 12.04, packages, etc. cannot have come from Debian Squeeze.

So then what other Debian Stable is there for Precise to be based on?

That part of that answer is wrong.

LTS is based (mostly) on debian-testing

From https://wiki.ubuntu.com/LTS:

• We are more conservative in our package merge with Debian, auto-synching with Debian testing, instead of Debian unstable.
• [LTS is not] Cutting Edge: Instead of doing an automatic full package import from Debian unstable, we will do it from Debian testing. The benefit we gain from not introducing new bugs and/or regressions outweighs the new features and/or fixes we often get from unstable.
  • We reserve the right to selectively pull in updates from unstable, if we believe the stability of the package in Debian is better than what is in the current Ubuntu archive.

The obvious corollary is that non-LTS releases are based, for the most part, on Debian unstable.

This appears to have changed recently per https://wiki.ubuntu.com/LTS: "Starting with the 14.04 LTS development cycle, automatic full package import is performed from Debian unstable"

https://web.archive.org/web/20131030153921/https://wiki.ubuntu.com/LTS