That says they were required to cancel it.

>But in another text message seen by Schwartz and Cramb an hour before their talk, the same Salesforce executive told the speakers that they should not announce the public release of the code, despite a publicized and widely anticipated release.

If the executive told them not to announce the open-sourcing, then he was expecting them to speak, just not that they would announce the open-sourcing. My guess is that they did not acknowledge that first request, and thus the executive told them half an hour later to cancel all. So the original issue of confrontation was about the open-sourcing, not about the speech itself.

If they missed the first text there's a good chance they might miss the second text as well.