"zero ohms" is very tricky to measure because of contact resistance: your probes aren't superconducting, and the thin contact surface between your probes and the metal itself isn't superconducting either.

Whereas the Meissner effect is unique to superconductors. It just has to be distinguished from diamagnetism, and there are materials which are strongly diamagnetic but not superconductors.

Build a loop out of a superconductor, induce a current in it, and then measure the resulting magnetic field after a couple of minutes.

If the field is still there, then you have a superconductor.

Because the levitation thing works with tiny fragments too whereas the 'zero Ohms' measurement only works when you have enough of it to form a long enough wire of it that you could tell the difference between 0 and 0.000001. Measuring very small resistances is usually done by letting a known current run through them and then to measure the voltage across the sample using known resistance leads. For a superconductor that has a tiny available sample which is already conductive even if it is not superconducting is fraught with error and may well give you the wrong answer due to a tiny measuring error.

But the Meisner effect is a unique signature of a diamagnetic material and will provide you some evidence even if the sample is tiny. So I understand why they have not yet resorted to other measurements, if there is no Meisner effect you don't need to continue with the hard work of trying to make a wire (which may well be a serious challenge for this stuff, the yield will have to come up significantly before that's a real possibility).