This is not entirely true.
What I said is true. You can download the code, make changes to it, and recompile it. I never said you could rewrite everything in it. You could completely rewrite networkmap if you wished to do so (to take on the example you are alluding to).
Folks complain about Client List every day. You know more than anyone else about Asuswrt. Can you fix it?
Networkmap is closed source. It used to be open sourced, I fixed a ton of bugs in it in the early days, but with 382 Asus made it closed source because they started to leverage proprietary BCM and TrendMicro APIs within it.
Personally it has always been reliable for me (except when it was randomly crashing, which was some of the bugs I initially fixed back in the day) or for the customers for whom I manage an Asus router. I know that some people with issues were using a subnet different than /24. At least back in the day, networkmap was hardcoded for /24, so it would miss clients if you used, for example, a /16 subnet. Some people with issues were also using repeaters of some sort, which would only expose the repeater's MAC to the router rather than the client's individual MACs.
Networkmap-related complains are far less common these days than they were back in 2014-2016.
Initially, networkmap's behaviour was fairly straighforward.
1) Send arp pings to the whole /24 subnet
2) When getting a reply back, start polling various services on that client to extract information (i.e. polling NETBIOS, LPR, SMB, etc...)
3) Add the client to the list along with the MAC, the IP and determine the type of device based on the polled information (nowadays they also leverage bwdpi to better identify devices I believe).
I don't know how that behaviour has changed since it was closed.