This was the first macOS release from Apple that bundled curl. It was a complete surprise to me as well when I realized this had happened. Nobody had told me about it ahead of time. I don’t even recall anymore how I figured this out, as I’m not a mac user and I’ve never had any direct contact with the peeps at Apple Inc who were and are responsible for shipping curl on that platform. Not before then and not after that either.
The general consensus at the time seemed to be that Apple replaced Wget with curl due to licensing reasons as Wget had been included in their previous OS X release. Wget is licensed under GPL and curl comes under an MIT-like license. I’m not sure if they shipped libcurl too already at that point.
Apple downloaded our source code tarball, built curl from it and shipped it. (They also made the exact code they used available.) If I remember this correctly, they built curl against OpenSSL and shipped a version of that as well in the beginning.
In later macOS releases (you know they later changed the name of their OS from being Mac OS X to macOS), they’ve switched curl to use different TLS backends over time. At one point they converted over to using Secure Transport (their own native TLS library) only to somewhat later switch again and then to libressl – which is what the current most recent macOS version uses for curl’s default TLS backend.