That's what's implied when you say 802.11b/g/n rather than 802.11a/b/g/n or 802.11ac. Both 802.11a and 802.11ac are 5Ghz-only protocols [0], and 802.11ac implies[1] support for 802.11a/b/g/n. :)
[0] And 802.11b/g are 2.4 Ghz-only protocols.
[1] Or it might dictate support, I haven't read the text of the standard.
It only is 5Ghz when there is a 5GHz radio in the device. I gather that world-wide 5Ghz operation not-infrequently requires the ability to do DFS. [0] This is an added engineering and testing burden that -I guess- many companies just don't want to undergo. [1]
I've spent an absurd amount of time looking for and at WiFi network adaptors. Devices that advertise just b/g(/n) support contain only a 2.4Ghz radio 99.9% of the time.
Look at the first-gen Chromecast, the 3G Nexus S, or the 2012 Nexus 7. They all advertised b/g/n support, and all operated only in the 2.4GHz band.
[0] There are regulatory domains in which the majority of 5Ghz channels require DFS and/or transmit power control.
[1] You'd think that all the engineering work would be done already, and only the testing would be left, but who knows?
