It always was - the initial point of Ubuntu from a technical standpoint (there was some mumbo-jumbo about togetherness & whatnot as well as the technical decisions) was taking Debian but compromising a little on some of Debian's hard rules in favour of newer versions and some other tweaks for friendliness. That is why the initial versions were based on Debian:Testing with bits of Debian:Sid where there were security or stability concerns in that branch (back then Testing could lag a noticeable amount on security updates, compared to both the stable and unstable branches), rather than starting with Debian:Stable.