I saw this the other day and thought surely if the link is under extreme high load then there is the possibility of dropped packets, but I don't know enough about settings on that particular router etc to be able to contribute anything useful.
Jelv & Weaver are actually correct... the ISP is supposed to rate limit to match the IP profile or line rate, but the only ISP's where we have seen evidence of them doing this is with AAISP and Plusnet.
It's several years ago now since I read the docs, but from memory basically it is both to stop bottle necks on the BTw backhaul and also to help reduce the possibility of dropped {ATM} packets at the DSLAM. eg if the download is coming down the backhaul at say 1Gbps (or multi Gbps) and suddenly is restricted to say xDSL 10Mbps DSL then you can end up with a bottleneck and dropped packets at that point. I don't know if this also applies to G.fast, but certainly it was documented for FTTC and ADSL. I'd need to do some research and double check if its something to do with ATM or not before I start chipping in my 2p.
The takeup rate for g.fast is quite low... if he is sharing a BTw backhaul with only a couple of g.fast connections then he possibly has a nice big fat pipe practically to himself right up to the OLT. BTw only police the amount of bandwidth traffic passing through the ISP host link and not the speed - which is why they recommend the ISP rate limits before downstream traffic hits their backhauls.
I dont know how the IP profile for g.fast works... or even if there is one but presumably there must be.
----
ETA can't find the documentation right now.. searching brings me to here
https://kitz.co.uk/adsl/IPprofile.htm#why_IP_bRAS_profile