Why are electric bikes ranked on torque ratings?
Addressing just the actual question as stated to avoid a 3,000 word essay:

With this in mind, how to interpret the advertised torque for a bike with a mid-drive hub, since it's heavily dependant on the transmission?

The advertised torque on a mid-drive bike will tell you over how large a band of cadences and how steep a hill the bike can actually provide the advertised assist level in watts.  (generally 250W equivalent mechanical rider input at the crank.)
A low torque bosch active line motor might only provide 250W of equivalent mechanical input assistance at exactly 70rpm on flat ground, whereas a Performance CX can provide that assist level at much lower cadences and on much steeper inclines.  The manufacturers try to reduce this to a simple number because it's consumer marketing.  (and also the drive system manufacturers don't sell you a final assembled bike, so they don't know what final numbers will be at the drive wheel in the installed system.)
The marketing is targeted to consumers buying finished bicycles made out of mass market bicycle parts.  In reality the transmission is always going to go from a bit over 1:1 to around 36:11 to 38:11 and the drive wheel will always be about 26&quot; to 29&quot; outer diameter, these things aren't just infinitely variable to get whatever outcome you want.
Certainly if it were all made out of infinitely ideal &quot;high school physics&quot; components you could use any combination of motors and gearings to get any torque number you like, but these things have to be made out of real materials that don't strip teeth or snap in half or overheat when spinning at the speeds that people's legs actually turn, so there are a lot of engineering compromises to be made.