I think getPageLinksInternal returns all links - internal and external. The "internal" in the method name is because it is called by getPageLinks:
https://github.com/libreliodev/android/blob/master/src/com/artifex/mupdf/MuPDFCore.java#L480
You can iterate through the array of LinkInfo you get and check for "instanceof LinkInfoExternal" to find the external links.
Example here:
https://github.com/libreliodev/android/blob/master/src/com/librelio/lib/utils/PDFParser.java#L42