Are you 100% certain on that?? The reason i ask is because I think that still needs some clarification or like some other TP-Link devices the spec varies slightly by region. If you look at the images at your link there is no dual wan or E-WAN port on the device (port 4 LAN and being able to bridge that to a WAN doesnt count, you can do that on basically any modern device) and the 63138 chipset i believe is dual WAN by nature (though i spose they could just not bothered soldering on a second port).
The VDSL spec at that link is also light. (no G.INP or G.fast mentioned or listed)
Either way both chipsets are more than good enough for many years to come, this country is not gonna see G.Fast or similar in mass for at least another 5+ years. So buying for a 63138 device for that feature right now is not worth it IMO. Even more so as BT will probably bar-steward-ise the spec of that format by the time it rolls round anyway
I personally got my info from here...
http://www.hwupgrade.it/forum/archive/index.php/t-2704218.html
where it seems its the LAN/WAN chip that is 1Ghz (IE BCM4709 1GHz, dual-core) not the DSL chipset.