But he denied and disobeyed.
meaning, he (Fir`awn) rejected the truth and opposed what Musa commanded him with of obedience. So what happened with him was that his heart disbelieved, and Musa (i.e., his call) could not internally or externally affect it. Along with this, his knowledge that what Musa had come to him with was the truth, did not necessitate his being a believer in it.
This is because recognition is the knowledge of the heart, and faith is its action. And it (faith) is to comply with the truth and submit to it.
Concerning Allah's statement,
ثُمَّ أَدْبَرَ يَسْعَى