[x] There exists no Z such that Z is since X and Z is better than Y.

I note in passing that "Y is strictly the best thing since X" means something just slightly different.

I was briefly confused by this until I realised you were using "strictly" to modify "best" rather than "since".

I do keep wondering if part of the problem is a confusion over whether the "since" is strict, i.e. a confusion between "Y is the best thing in the interval (time of X, now]" and "Y is the best thing in the interval [time of X, now]".

I almost wrote "Y is the strictly best thing since X", but decided it was horrible, horrible English.

You're right. I now realise I was assuming strictness for "since" but not "best". Ooops.