After posting, I realized an obvious difference: I'm now building the Apple Silicon version, not the Intel one, so the "existing binary" is running under Rosetta, but the compiled one is running apple silicon.

So, the ultimate answer proved to be totally different. After agonizing amounts of diagnosing, trying new configurations, I finally realized that there are two error messages up there: Reason: no suitable image found.  Did find:  file system relative paths not allowed in hardened programs The first one turned out to be correct; there was, in fact, a dylib missing from my build. (One for x86_64 arch) The second message is completely wrong, and led me down a long irrelevant path.