Well, there are two parts to this question.
First, the chairman cannot dictate policy and has few rights beyond those of any other councillor. And councillors cannot act except through meetings of the council and its committees, or on the instruction of a properly made decision in a meeting.
But the chairman is merely stating the law. A council cannot make a decision except in a properly constituted meeting where due notice has been given to the public, and can only decide on the matters described in the published agenda. Only councillors who are present at the meeting may vote.
So any purported decision reached by email or any other method outside of a council meeting is invalid.
ADDITION: At first sight, these rules may seem bureaucratic. But I think one needs to bear in mind the underlying reason for them. It is democratic accountability, something that I regard as extremely important for healthy local government. With rare exceptions, council meetings must be held in public, so that people can see for themselves what is going on. The requirement to publish an agenda some days before a meeting is to allow citizens to know what may be decided, and make representations to councillors if they wish. That is also the reason for insisting that a council can only decide matters that have been clearly identified in the agenda. It may seem tiresome having to check with statutes for the rules governing council decisions, but really if you asked yourself whether the council is giving electors a fair chance to be involved with those decisions, you would most likely come to similar ways of working.
