Because heresy is not a matter of a point of view of an individual…it is a specific delict, which is both treated and adjudicated by Canon Law.
The matter is moreover further elucidated by
Unitatis Redintegratio when the Council Fathers said
This Sacred Council is gratified to note that the participation by the Catholic faithful in ecumenical work is growing daily. It commends this work to the bishops everywhere in the world to be vigorously stimulated by them and guided with prudence.
Thus laity’s pronouncement on what is and what is not conformed to the Church’s mind on ecumenical matters must do so by referencing the mind of the Pope and of PCPCU first and foremost and then the Bishops.
For Americans, this can be done through the ecumenical office of one’s own diocese or through the Secretariat of Ecumenical and Inter-religious Affairs of the National Conference of Catholic Bishops.
usccb.org/beliefs-and-teachings/ecumenical-and-interreligious/
And, as far as the laity are concerned, this is made all the more explicit in the dispositve document *The Directory for the Application for Principles and Norms on Ecumenism:
*
Catholics need to act together and in agreement with their Bishops. /…/ They should also have accurate knowledge of the other Churches and ecclesial Communities with whom they are in contact. Careful note must be taken of the various prerequisites for ecumenical engagement that are set out in the Decree on Ecumenism of the Second Vatican Council.
So, no, in fact one may not say that someone is speaking heresy for articulating a position that the Holy Father, the Holy See and the bishops have said is not applicable to the canonical delict.