No, the internal memory buffer does not keep the disk from being finalized.
In most cases, the recorded disk is "left open" to allow additional recordings to be added to the disk (space permitting).
You will have to look at your user guide (I am not familiar with your exact recorder, but it should be similar to my Philips recorders), there should be a menu option to explicitly finalize the disk. The finalization process transfers the TOC information from the memory buffer to the disk.
After finalization, no additional recording can be made to the disk.