No breach, boss.
"The Parties agree and undertake to enter into mutually agreeable definitive agreements (“Definitive Agreement”) and any other documents necessary for the closing of the Transaction, within 90 days of the execution of this LOI."
So the LOI (which is a very basic agreement) states that they have 90 days to agree to another agreement called the definitive agreement (which is a more detailed agreement). They already agreed to the definitive agreement within that 90 day period. If the LOI said they have 90 days from the signing of the LOI to agree to and close the definitive agreement, then we'd have a different story.
What no one knows is the closing date that's specified in the definitive agreement.