This mirrors my experience.
It's worth noting that TeX was developed in the same time period that the details of lexical scope were being nailed down by Guy Steele in the Rabbit compiler for Scheme. It's not that TeX is an ad hoc system; it's more the case that people didn't actually know how to implement a better system at the time.
'People' in this case were Don Knuth (TeX) and Leslie Lamport (LaTeX). Both are Turing Award winners.
That's true. Do you know who else won a Turing Award? Tony Hoare.
What is Tony famous for? Well, lots of things, including his very important comparison sort algorithm Quicksort, but, in this context how about the Billion Dollar Mistake ? That's a pretty nasty booboo in many programming languages for which Tony blames himself because it was his idea.
Like your parent said, TeX shipped a long time ago and we learned a lot since then, it is not a surprise that we know how to do better today, in fact it would be a serious black mark for Computer Science if we couldn't.
Which means what, exactly.
The details of lexical scope where defined in Algol 60 (1960), nearly 2 decades before Rabbit (1978).
People did know how to implement things back then, and TeX is a great example of that. It is just our definitions have changed over the years of what we consider better.