Wa akeedu kayda
And I (too) am planning a plan.


Allah doesn't mention Inna in this verse because, Inna hints at some sort of effort that's required.

Allah simply says - meaning, I have a plan (singular) which takes over all their plans.


We also notice that throughout the whole surah, Allah hasn't spoken directly to the disbelievers. He's simply spoken to Allah's Messenger, and then through him to the believers. By not talking to the deniers, He's humiliating them i.e. they're not even worth talking to.

Imagine a teacher is criticizing another student for his bad behaviour, but he's talking to the other pupils instead of him directly. This is going to make the bad student feel humiliated. The same is the case in this surah, and in other surahs like it too.