It's a daily thing we all do: decide if this problem is better solved by a big chunk of code that is probably well tested but probably satisfies a bunch of requirements and other constraints or a smaller chunk of code that I can write or vendor in and has other advantages or maybe I just prefer how its spelled. Sometimes there's a "right" answer, e.g. you should generally link in your TLS implantation unless you're a professional TLS pereon, but usually its a judgement call, and the aggregate of all those micro-decisions are a component of the intangible ideal of "good taste" (also somewhat subjective but most agree on the concept of an ideal).

In this instance the maintainer of a useful piece of software has made a choice that's a little less common in C++ (totally standard practice in C) and it seems fine, its on the bubble, I probably default the other way, but std::function is complex and there are platforms where that kind of machine economy is a real consideration, so why not?

In a zillion contributor project I'd be a little more skeptical of the call, but even on massive projects like the Linux kernel they make decisions about the house style that seem unorthodox to outsiders and they have their reasons for doing so. I misplaced the link but a kernel maintainer raised grep-friendliness as a reason he didn't want a patch. At first I was like, nah you're not saying the real reason, but I looked a little and indeed, the new stuff would be harder to navigate without a super well-configured LSP.

Longtime maintainers have reasons they do things a certain way, and the real test is the result. In this instance (and in most) I think the maintainer seems to know what's best for their project.