My counter-theory sir is that the Terrans had a much more disparate TL at contact. Jump drives, sure TL9...but most other systems TL 10 and even 11. So when they hit Vilani, the contest was closer than it would seem. ...
Maybe, maybe not. Terrans have a long history of becoming really clever and innovative when they get embroiled in war. Must have been a hell of an incentive to get out there and find out there was someone bigger and stronger than you, and - most important - the Vilani by all accounts really didn't take them seriously until it was too late. Underestimating your foe has brought many a great power to its knees.
The Terries benefitted from a change-oriented culture and a smaller population base - compared to the Vilani - in which to disseminate technological change, allowing them to integrate improvements much more quickly than their conservative, sprawling counterparts. And, half the battle in science and tech is knowing something can be done.