Placing yourself at the top of the enmity list, is not a way of initially pulling aggro.

You might have aggro for a split second, but the moment anyone else generates any enmity, you will lose aggro. This will occur within a split second as someone else's attack connects after your Frog Legs.

Frog Legs is a enmity catch up mechanic. If you're way behind on enmity and you need to be on top, you start with Frog Legs to put yourself on the same level as the highest enmity. But in order to gain an enmity lead, you need to use something that generates enmity, which Frog Legs doesn't do.