I thought about what makes a Megadungeon, and here are my thoughts:
1.) The PCs will spend many levels adventuring in it - at least 7.
2.) There is a thematic element to it that ties it all together (all made by the same villain, an extended set of ruins that wer all once part of the same city, etc..).
3.) The dungeon involves the physical structures of a dungeon (walls, traps, monsters).
I considered 'Dead in Thay' from Tales of the Yawning Portal when thinking about the criteria here. It is a huge dungeon. It has the same thematic tie. It has the physical structure of a dungeon... but it only is supposed to take 3 or 4 levels to complete. Is that big enough to be a megadungeon? I thought not.