Don't you think it's a bit odd that the example program uses short-circuiting booleans as conditionals and uses the fabled downto operator?[1] The article's not teaching how to program, it's just showing you how to set up THINK C -- the intro suggests that the target audience is programmers ("I can see [...] that you, too, are a programmer.")

[1] https://stackoverflow.com/questions/1642028/what-is-the-oper...

If one’s going to argue about choice of first programs, a proper Hello World on classic macOS requires the use of the Mac Toolbox. Using the bundled Think C console isn’t really producing a Mac-native program.

Where's a good place to find a guide to that?

The definitive reference is Apple's own Inside Macintosh. Many parts of that are still available on the Apple developer web site, e.g.

https://developer.apple.com/library/archive/documentation/ma...

https://developer.apple.com/library/archive/documentation/ma...

For a more directed and more C-oriented walkthrough, the Macintosh C Programming Primer was a pretty common go-to. It's been out of print for ages, of course, but there's a PDF available at:

https://vintageapple.org/macprogramming/pdf/Macintosh_C_Prog...

You can search for "Toolbox" and "Tool Box" here:

Vintage Macintosh Programming Book Library - https://vintageapple.org/macprogramming/