Yes, instrumention amps are often used for low-voltage amplification...primarily because the topology enables the elimination of impedance-matched input termination with the distinct benefit of minimizing parasitic load on the UUT. Apparently, readily-available integrated instrumentation amps on the market are designed for relatively low-bandwidth applications as a quick DigiKey parametric search suggests, but the canonical topology itself isn't fundamentally bandwidth-constrained...which is part of the reason why I pointed to max GBW results from the search and left it at that.
No, that's not what I'd be looking for, especially in response to your question with the specific constraint of "an integrated version of a differential amp"; given the sensitivity of the in-circuit measurement application being considered, this effectively suggests integrated instrumentation amp. With your suggestion of what I should be looking for, how would you go about input termination without loading the UUT to the detriment of SI?
The author does indeed briefly discuss his prototypes and even released his Eagle design on GitHub[1] with still more notes.
For the record, I didn't downvote on first response, but your "oh-so-brillant-ones" snark may have earned one.