The maintainer just stated it without further explanation. Searching for "HOMEBREW_NO_ANALYTICS" does reveal some other references[1], and it seems that there are some other places where this may need to be added such as "Library/Homebrew/env_config.rb", but I don't know anything about homebrew.
I do think it's very much a minimal-effort PR. It's hardly a PR at all: at the very least the tests and documentation should have been updated, and actually writing out some more rationale would help as well. I certainly wouldn't send a PR like this, at least not without the text "this is a proposal, and if accepted I will update the tests and documentation as well".
Probably the homebrew maintainers have hard coded usage of “HOMEBREW_NO_ANALYTICS”. The PR likely only fixes one instance. Lack of unit tests in PR support this hypothesis.
Could be wrong though and the maintainer simply needed a vague excuse to not merge it. Haven’t completed a deep dive. I couldn’t be bothered to compare repository at the time of PR 2 yrs ago on mobile.