I am not old enough to know how resource forks were implemented on Mac OS but this is definitely not the case today. Resource forks are implemented (or maybe "emulated" is a better word to use? Not sure how much effort is put into them) as random-access. You can use POSIX APIs to interact with them (using _PATH_RSRCFORKSPEC) and these are typically faster than other interfaces.
Back in the day, you used the Resource Manager to open a resource fork. The resource manager provides functions to load individual resources, query which resources exist, and add or modify existing resources.
The Resource Manager made it to Mac OS X as part of Carbon. The main part of Carbon is gone, but a part of it called CarbonCore survives, and that contains the resource manager. If you dig through the docs, you can find it. It was deprecated in 10.8 (which seems really late… the writing was on the wall about resources back when 10.0 hit).
The modern resource manager functions in CarbonCore I think just use the POSIX API underneath. Undoubtedly, there’s some test suite at Apple that makes sure it works correctly. Also undoubtedly, there’s some application vendors who wrote code using resources in the 1990s and still has some of that shipping today.
