"Legacy 802.11a/b/g devices do not support the client match feature. When client match is enabled on 802.11n-capable devices, the client match feature overrides any settings configured for the legacy bandsteering, station handoff assist or load balancing features. 802.11ac-capable devices do not support the legacy bandsteering, station hand off or load balancing settings, so these APs must be managed on using client match"
Quoting the above statements from the link you have provided. What i understand from this is that client match was there even before in the form of options like legacy bandsteering, station hand off or load balancing. Now as the 11ac APs do not have these options hence client match (with few more enhanced features) was introduced. Make sense :)
Thank you cjoseph. For the cleanest and quickest responses, i like Aruba Networks more than other wireless vendor :)