But even then it didn't act like a comet. No carbon monoxide, carbon dioxide, or dust was seen, which are very common in comets.
On top of all that, it was brightening and dimming with a regular period, indicating it wasn't spherical (and as it tumbled we saw a different geometry reflecting sunlight back to us). That's common enough, but it must have been a really weird shape to explain the extreme change in brightness seen, either shaped like a pancake or a cigar!
Lots of attempts were made to explain it. It's a hydrogen iceberg (unlikely). It's a fractal snowflake (cool, but also unlikely). It's a spaceship (no).