you are right about measuring the impedance with a DC resistance measurement, bur the measured resistance gives a hint in my opinion:

The coil itself is a series of the wire resistance itself and the ideal coil with it frequency dependant resistance.

So the DC resistantance measurement of 6 Ohms directs it to an 8 Ohm tweeter, This should be true under the assumption that the 6 Ohm measurement is correct done with proper, clean probes, clean contact poles at the tweeter, a low error of the multimeter.

A speaker driver will only present its nominated impedance at one specific frequency. Any other frequency and the impedance will change accordingly. Measuring the impedance is a rather complex job but - in popular terms - the nominated impedance can be seen as an average figure derived from measurements over the drivers nominated frequency range. Outside the nominated frequency range the drivers impedance will be far off (and the driver won't play much).

I agree that a DC reading of apprx 6 Ohms could point towards an 8 Ohms driver, but don't take this as a guarantee. Factors like the resistance of the (particularly in tweeters) very thin copper wire can play a role.

Many 1970s and 1980s B&O speakers have "Impedance 4-8 Ohms" at the label at the back and lots of people ask themselves, are they 4 or 8 Ohms then? Well, they are 2-way, 3-way or even more-way speakers. The total impedance depends on the frequency you put into the complete speaker and which - and how many - driver(s) are handling the given signal. In other words, the impedance curve goes a bit up and down for each crossover frequency,

Saying that a speaker is simply 4 Ohms or 8 Ohms would be less correct than stating a range like f.e. 4-8 Ohms.