Yes, I’ve read that thread before - I have the (sometimes useless) ability to read way too many interesting issues 
Let me quote you right back at yourself though:
From Subnormal number - Wikipedia
well-behaved software should save and restore the denormal mode before returning to the caller or calling out to unsuspecting library/OS code
From here.
So I’d argue that the library not resetting the flag was faulty behavior and a library compiled with -Ofast shouldn’t affect the caller either way 
(especially not influence how julia compiles its own code, which is the topic here, instead of getting incorrect results from correctly compiled code, as in that issue)