If you can break the ECDH, you can figure out the key that was calculated. However, if other unknown data (e.g. past ECDH outputs) is hashed together to generate the key (like Signal does, as a very rough description), it's only post-quantum secure if the adversary ever misses a single message.
Other than weird saved-by-the-bell corner cases, the answer is "No".
If you can break the ECDH, you can figure out the key that was calculated. However, if other unknown data (e.g. past ECDH outputs) is hashed together to generate the key (like Signal does, as a very rough description), it's only post-quantum secure if the adversary ever misses a single message.
Other than weird saved-by-the-bell corner cases, the answer is "No".