Okay, this is how it works. After 180 days (for Official Plan amendments) or 120 days (zoning by-law amendments), an applicant is entitled to appeal its application from the "neglect" of Council to make a decision. Sometimes such appeals are referred to as "private appeals". Sometimes developers will strategically trigger private appeals, even in instances where there remains a good chance that Council will eventually support the proposal, for a variety of reasons. The most common reason is the expectation that third parties are likely to appeal any Council approval anyway, so the developer tries to speed things up by getting the OMB process underway so that they at least have a prehearing date scheduled by the time Council makes a decision (in other words, the third party's ability to delay the project is reduced). Another reason is that the OMB process can sometimes force the City to act more quickly than it sometimes would - the scheduling of an OMB prehearing or hearing date typically obliges planning staff to report to Council beforehand so that the City has a position at the Board.
In the case of private appeals, the applicant is required to send out the notices for the prehearing. That's why these notices are coming from the developer's law firm.
It's worth noting that back in July, Council didn't technically approve the rezoning for the One. The application had already been appealed to the OMB, so that decision was no longer Council's to make. Instead, Council instructed the City Solicitor and other appropriate staff to attend at the OMB in support of the proposal.