The drills I am hoping to use are designed for 120k RPM for best life.
This is a big problem because if I have a correct understanding of your objective the spindle is the least of your worries. Micromachining isn't just limited by the spindle, any errant motion will snap the bit instantly. Everything from the toolholders, linear guides, ballscrews, servos, and machine frame must be chosen for zero play and high damping.
I totally get it. My machine is 5,000 lbs and the motion control is suitable for entry-level micro machining - end mills down to about .010" if the RPM is available and the programming is perfect. It is true that the slightest imperfection anywhere in the rotating assembly is a show stopper. Any un-commanded motion in the workpiece is a show stopper. 60k RPM is not a place where cutting corners will have a good result.
As a reference - the 90k RPM spindle I am looking at costs about $5,000. It's runout is lower than I can even measure in my shop, in the low 'millionths' territory while rotating [verified of course]. It goes into a larger ATC spindle like the one on my machine which is trammed extremely well. That level of precision is suitable for extremely small tools. Not sure what to expect from this eBay option - but obviously it will not be that good. My guess is that for engraving and drilling, it would allow fairly small tools to survive a reasonable amount of time when it is new. I would not have much expectation that it would last very long though where the TIR just grows and grows until it becomes a tool killer.
The runout spec claimed is probably from the spindle that they copied to make this one - which is near the claimed spec. At least not while it is spinning at 60k RPM. I don't know of anyone hobby or professional that has the capability of measuring 50um (or whatever the claim is) while it is rotating. That requires some fancy non-contact laser metrology to confirm. So - if no one can prove it, why not exaggerate it, lol