It’s a reference to a claim developers Image-Line made that they’d add native Mac support “when Hell freezes over.” The comment at the time wasn’t so outrageous: FL Studio had been built a Windows-native development toolchain that made porting unthinkable.
The “Hell freezing over” bit you’ll see a lot around this release. At age 20, the software still includes lifetime free updates – and a bunch of new features, including freezing of audio, and Hell freezing over. FL Studio aka Fruity Loops has hit a version the developers are dubbing FL Studio 20.