Is Emperors Divide choosing or not?

Yes, Emperor's Divide involves choosing.

According to the official FAQ for Emperor's Divide:

• It involves choosing: When you play Emperor's Divide, you choose which specific units to move as part of placing the spell on the chain.
• It counts as choosing/targeting: Because you select specific units when placing the spell on the chain, this counts as choosing or targeting those units for the purpose of other card triggers (such as Dreaming Tree or Blade Dancer).

This is distinct from spells like Whirlwind, where unit selection happens after resolution and is not considered "choosing" in the same way.