The Church of England is the name for the Anglican Church in England only and he is speaking for the Church of England. The many churches in the Anglican Communion are autonomous.
The C of E might be the 'Mother Church' of Anglican churches worldwide but the Church in Wales, for example, will bless the marriage or civil partnership of same-sex couples.
As the law stands, same-sex marriages may not be solemnised by the Church in Wales. However, following approval at the Governing Body of the Church in Wales in September 2021, a Liturgy for the Blessing of a Same-sex Civil Marriage or Civil Partnership has been authorised by the bishops for use in each of the dioceses from 1 October 2021.
So the Church in Wales cannot consider same-sex marriages to be a sin if they bless them, surely?