The property currently checks the pointer size which, under 32-bit Mono, will be 4 and thus the property will return 'false' which is incorrect since the operating system is 64-bit.
The code to query the bitness of the OS could just invoke sysctl, in its simplest and most primitive form:
$ sysctl hw.optional.x86_64
I took a stab at it in https://github.com/mono/mono/pull/2422
*** Bug 5483 has been marked as a duplicate of this bug. ***
PR was merged.