because they need to have VDSL (FTTC) capable modems...AND because of the way Openreach have it setup with ISPs you must use their combo-kit
FTTC is just fibre-to-the-cab , from the cab it is not fibre, it is copper using VDSL which is Very High Speed DSL, just like ADSL but higher speeds, the HG612 is a VDSL modem, it handles the broadband connection, but it is designed to pass of the connection details to the ISP's router, the ISP router controls the HG612 (if you like) but the HG612 actually MAKES the connection - the HG612 does all the work.
that comment you posted is just referencing that regular modems will not work with FTTC which is true because of two reasons
1) most modems in routers may not handle VDSL...only ADSL
2) the way that Openreach have it set up the ISP's supplied router combines with the modem to make the connection live to the end user, so even if you have a VDSL modem you can not use it unless its hacked to work like the way I mention.
I have an unlocked HG612 so it works on ADSL (its both an ADSL and VDSL modem) and doesnt need to pass off the connection to another router, it becomes the sole host, now if its possible to do it for ADSL connections on the HG612 via modded firmware...I wonder if its possible to do it for VDSL connections?, but like I say if it was I would imagine it would be on Sky-User as the easiest way to do it, I just cant understand why its not possible to do it (if it isnt possible to do it) - probably be something to do with the ISP detecting whats connected and blocking it if their router isnt detected .
the way they seem to be doing it on SkyUser (with a very small selection of routers) is by fooling the ISP (sky) into thinking one of their routers is connected too the modem...so it looks like what I said above is probably right, like they wont see their router connected 'through' the HG612.
otherway around it is too have a 3rd router connected, turn off Wi-Fi in the sky router and connect another router via LAN with DHCP off and change its IP Address <- this way you can use ANY router you like for Wi-Fi and not be stuck to use just 1 or 2 that have been hacked like that TP-Link and the ancient Billion.
that TP-Link router has an average of 40Mbps and MAX of 60Mbps for wifi throughput on that website you posted on other website... the speeds seems a little low to me...but hey, maybe they are right?
anyway, it looks like you can either use the hacked routers which limits you to a couple of models - or you can use a 3rd router and can use any router you like