Apparently, just being defined is enough.

d80edb5c · Deucе · 605880b5 · d80edb5c
Commit d80edb5c authored 2 months ago by Deucе
--- a/src/syncterm/telnets.c
+++ b/src/syncterm/telnets.c
 /* Copyright (C), 2007 by Stephen Hurd */

-#if defined(__STDC_NO_ATOMICS__) && (__STDC_NO_ATOMICS__)
+#if defined(__STDC_NO_ATOMICS__)
 #error Support for stdatomic.h is required.
 #endif