Actually, thinking about this again, I would say it either implies "X was good and Y is nearly as good" or "X was good and Y is good too, but no implication is intended as to which is actually better". I am, in fact torn between these two. If you say that [X flavour] crisps are the best things since pizza, I think that probably implies that pizza is better. Or else surely you'd go further back and say that [X crisps] are the best thing since roast beef or something! :P

"X just marks the start of the time interval under consideration, and no implication is intended about whether it was even particularly good at all"

This would work too!

I've been trying to think of a particularly good example of that usage. Best I've come up with so far is that one might just about plausibly describe something as "X is the biggest step towards international harmony since World War II", intending to imply not that WW2 was itself a step towards harmony but that it wiped out the effect of all previous such steps, making X the biggest step whose effects we're still feeling.

This is, essentially, what I understand the phrase to mean in general. But as usual, you've just worded it better than me. ;o)