Hmm. You make a good point about the core dimensions. I'm not sure if those dimensions are the ID and OD, or maybe the OD and height? Joule Thief circuits are fairly robust, so they are not super picky about having an exact core size or a particular number of turns to work. I might just look up other joule thief designs on the internet with better documentation and maybe experiment with whatever they used.
As for frequency, I believe F29 is a type of nickel-zinc ferrite material optimized for use at several MHz, but from what I can tell most joule thief circuits run in the tens of KHz range or so, so I don't know that it is necessarily the best material to use, anyway. These circuits are not very precise about what frequency they run at, and it depends a bit on the type of core used and the numbers of turns. A while back, I wound several dozen turns of 37 gauge wire around some random ferrite I found in a CFL light bulb ballast and had it oscillate just fine in a Joule thief circuit. It ran at such a low frequency that one could audibly hear the coil whining.
I think the article specifies the turns individually for two separate wires wound on the core-- one wire with 40 turns and one with 60.