The current documentation of both functions reads very similar to each another:
Both are static members of the same class, with one of them decorated inline, so I wouldn't think they are separate implementations with equivalent functionality you often find across various Delphi classes (although examples within a common class do exist in Embarcadero's standard library).
Specifically, I can't come up with a case where a path is a valid UNC path but not a rooted UNC path. So what does IsUNCRooted even mean?
IsUNCRooted only checks if the parameter starts with an UNC sequence, while IsUNCPath also checks the rest for valid path names. So a valid UNCPath is indeed also UNC rooted, but not always the other way round.