I just reviewed data for 12 of the top 50 players who aren't in top 50 guilds. Looking at that, it seems to bear out a bunch of these players do prefer to play solo. For example, a player has contributed 14 mil but guild statues are only level 26. In another example player has donated 3.5mil gold this week, yet all tasks are not completed. Another is level 1153 player who's donated 60mil to his guild, but statues are only level 67. A level 1164 player who donated 86mil but statues are only level 70. In addition to these players who appear to be solo, there are a couple that seem to be very new (within the last couple weeks).
I also did find a few players on the leaderboard that weren't part of top 50 guilds but were part of the same guild. And the gold contributions and statue levels seem to suggest that these are high level guilds that are hovering just outside the top 50, and/or are growing pretty fast.
Some of these accounts could be bots, how many is anybodies guess. However, the data seems to show legit patterns that would suggest that (1) the leaderboard is working/displaying accurately and (2) there are many non top guild players. There is no evidence to suggest "fake data created by the devs" (unless you believe the "conspiracy" also includes putting together complete and realistic history outside of what is visible in the game, and in that case I can't help you).
As for @Saltypatra's comment, hopefully she replies to clarify. But my guess is she meant they were in guilds, just not high performing ones.