"Pre-existing assets that were not created specifically for the contest are allowed."
Basically, only resources that are made SPECIFICALLY for your entry must be made within the contest time period.
Viewing post in Clarification of allowed assets
No, they would not be allowed. They are unique to the contest entry.
Basically if someone can show you created a piece before the time period started (say by showing a blog post from before the contest that showed you showing off the piece in a screenshot), it is on you to prove that it was either made for another project (because you released the project, showing that it was made pre-contest for another game), or because you had released it to the public and it was just meant to be made as a generic piece and not intended to be used solely for your contest entry.
While technically it could happen that you had made a sprite for another game, but then decided not to make that game, you have no way to prove it, so the easiest way to avoid the issue is "is this unique to this contest entry."
Remember, these rules are designed to try to prevent people from getting huge head starts on others.