I disagree with the first point. A function such as std::distance can indeed select the best algorithm, by making a function call using the tag. That is, distance(a, b) calls distance_impl(a, b, cat(a)). Or maybe it instead does:

if constexpr (derived_from<cat_t<decltype(a)>, random_access_iterator_tag>)
  return b - a;

This selects the optimal implementation without a function call.

Hmm, I guess you could view it that way as well. The way I see it, it still wouldn't be that the function selects the algorithm per-se, but the if constexpr block would end up calling the desired function directly by discarding all other options.

In practical terms, I'd also call if constexpr a form of "implementation selection" though. Perhaps the first part can be reverted and we can keep just 2.