The early ABBA releases where released as “Björn, Benny, Agnetha & Frida” and it was treated as a band name rather than a collaboration of four artists, for example they use the name even when Agnetha was replaced by Inger Brundin (live example https://www.youtube.com/watch?v=GgOIY0GyeN0).
About the name ABBA (from wikipedia):
“The first time “ABBA” is found written on paper is on a recording session sheet from the Metronome Studio in Stockholm, dated 16 October 1973. This was first written as “Björn, Benny, Agnetha & Frida”, but was subsequently rossed out with “ABBA” written in large letters on top.”
Later releases of both Waterloo and Ring Ring used ABBA instead of “Björn, Benny, Agnetha & Frida”, which shows intent that the artist consider them ABBA releases.
My take is therefore that on the Waterloo release they used both the new and old group name on the cover and that this combined name should be an artist alias for ABBA.