The PING ONLY discovering function, per the GTAC KB, only works in the legacy console and only works if the 32-bit version of Java Runtime Engine (JRE) is installed on the client system. If only the 64-bit version of JRE is installed, the legacy console ICMP/PING ONLY discovery function does not work.
As Zdenek stated, this feature will become available in a future release of ExtremeCloud IQ - Site Engine. It is not destined for XMC.