No, the application would not be POSIX-compliant, but it would be running on a POSIX-compliant OS. I was confused by the comment I replied to because it sounded like he was saying that POSIX compliance was more likely in "modern" applications, but if anything the opposite is the case.
POSIX compliance is much more common in consumer OSes nowadays, to the point that every major desktop and mobile platform is built on a POSIX-compliant kernel and runtime except for Microsoft's offerings. In that sense POSIX has taken over the non-Microsoft world, but now it's being used as a much lower-level compatibility layer for system-level frameworks rather than what it was originally designed for.
Even Windows has had multiple POSIX implementations over the years, they were just built as compatibility layers on top of the native APIs. So POSIX-compliant operating systems are almost universal at this point, but POSIX-compliant applications/daemons/etc. (all the programs that run on those POSIX OSes) are rarer than ever.
it sounded like he was saying that POSIX compliance was more likely in "modern" applications
Yes. Part of this is because more operating systems are approaching POSIX-compliance; but the number of patches we need to port random Linux code to run on FreeBSD has dropped tremendously over the past 20 years.
POSIX compliance is much more common in consumer OSes nowadays, to the point that every major desktop and mobile platform is built on a POSIX-compliant kernel and runtime except for Microsoft's offerings. In that sense POSIX has taken over the non-Microsoft world, but now it's being used as a much lower-level compatibility layer for system-level frameworks rather than what it was originally designed for.
Even Windows has had multiple POSIX implementations over the years, they were just built as compatibility layers on top of the native APIs. So POSIX-compliant operating systems are almost universal at this point, but POSIX-compliant applications/daemons/etc. (all the programs that run on those POSIX OSes) are rarer than ever.