There is a limit to what you can find from looking through the firmware, there may well be common drivers that work with more than one chip, or, perhaps in this case, someone decided to keep using an old file name with newer generations of hardware.
Looking at the AC56U firmware, there's also mt7510ptm and mt7510sar kernel modules (PTM as used with VDSL2, SAR probably an abbreviation for "segmentation and re-assembly", for ATM). The tc3162_dmt.ko contains strings with MT7510, MT7555, and 3262L1Afe3095 and 3262L1Afe60901 - it seems to have a lot of different things for different DSLAM chip vendors (as will other modems).
So I'm pretty confident it's a MediaTek DSL chip, but I can't tell which specific chip from looking at the firmware. Which exact chip might be provided from the running device, in some stats or logs or command output, or failing that, opening one up and reading what's written on the chip itself.