I've been told, many times, that probes can only be used by aerospace craft when they're on the ground. Which is confusing since they can be used by VTOLs in the air. However, Page 222 of Tactical Operations says that Airborne units can detect ground units using sensors and that the ranges don't change. Then it goes on to say that only works from a certain altitude or less and from a certain hex on a map.
Does that apply to probes as well as standard sensors? If not, why not?
Pg. 222 are rules for Hidden Movement rules, not rules for Hidden Units. Hidden Movement means you can't see it because something is in the way (a wall, fog, haze, etc.). Hidden Units you can't see because they are specifically hidden from detection (shut down, under sensor nets, etc.)
Since ASF generally have LOS, they don't really need the sensor detection rules, however they are there for added game play. An Active Probe on an ASF is still not going to see a Hidden Unit or get any other benefits of an Active Probe.
Does the altitude of 9 or less also apply to VTOLs?
I'm fairly certain we capped VTOL altitude below that already, though I don't have that errata handy. However, yes, it does apply.
If sensors function at altitude 9 or less, why can't ECM?
ASF ECM functions perfectly fine below altitude 9. It just does not operate the same as ground ECM so it has no effect on ground units.