Couple things:
What Trump did is something called “opening the door,” which means that by introducing a concept, you make it subject to being challenged by the other side.
Also: if you voluntarily repeat statements outside the scope of the privilege, then you waive it. Example: A client repeating to someone else what they had previously only shared with their lawyer waives the attorney-client privilege.
The executive privilege is intended to protect the deliberative process and highly sensitive matters such as national security.
But: if Trump is talking about these conversations openly, broadcast to the entire world on Twitter for crying out loud, then yes, that erodes the entire basis of maintaining the privilege in this instance and constitutes a waiver by Trump.
This entire tweet chain by Trump is a dumpster fire of privilege waivers.