Up until recently, the only option for FTTC was engineer install with an Openreach supplied modem (either HG612 or the ECI modem). In fact, the HG612 is a router and the ECI modem may well be a router, too, but Openreach supply these devices with a locked-down bridge configuration.
In effect, Openreach provided an Ethernet port and it was up to the end user or their ISP to provide router / firewall / wireless access point functionality using Ethernet connected equipment.
More recently, Openreach has offered engineer install with ISP supplied VDSL2 equipment, or self install (where the engineer carries out the work at the cabinet but leaves the installation at the end user premises to the customer).
For whatever reason, your Openreach engineer thought he was supposed to supply an Openreach modem as part of your installation. I suspect this was an error and that the intention was he used the TalkTalk supplied HG635, though it doesn't matter. You've finished up with an HG612 that isn't necessary if you choose to use the HG635.
So far as the leaflet goes, this seemingly reflects the historic position where use of an Openreach supplied modem was mandatory, at least in theory. This is now out of date, as ISPs are allowed to supply their own VDSL2 equipment.
If you are happy with the configuration you now have, you can put the HG635 in a drawer or return it to TalkTalk. I'd be inclined to keep it, as it's often a good idea to have the standard equipment used by your ISP to hand for troubleshooting purposes if you don't normally use it.
If you wish, you can disconnect the HG612 and replace it with the HG635. Best practice is to turn off the power to the old VDSL2 device before disconnecting it from the line, then wait at least half an hour before powering up the new device. This helps prevent unnecessary intervention by Openreach's DLM.