Not much more to it. llGetFreeMemory report 16000s bytes free at all times, llGetSPMaxMemory reports 16384 whether profiling is on or off. Same applies to pure Luau scripts, neither function returns meaningful values.
~~llGetUsedMemory and ll.GetUsedMemory appear to be ok.~~ <- Edit: redacted, they do not count memory used by tables or strings at all. A table with 10000 random numbers in it is reported as not using any memory, as well as a string with 256k(! that can't be right either) characters. Same applies to lists when compiled as LSL-Luau.