That’s not what they thought at the time they condemned him as a heretic, the letters they burned were because he was actively promoting it, supporting Sergius. He was condemned for “confirming” the heresy, he is counted as one of the leaders of this heresy:
[Answer of the Monothelites made at the Emperor’s bidding:]
We have brought out no new method of speech, but have taught
whatever we have received from the holy Ecumenical Synods, and from
the holy approved Fathers, as well as from the archbishops of this
imperial city, to wit: Sergius, Paul, Pyrrhus, and Peter, as also
from Honorius who was Pope of Old Rome, and from Cyrus who was Pope
of Alexandria, that is to say with reference to will and operation,
and so we have believed, and so we believe, so we preach; and
further we are ready to stand by, and defend this faith.
Honorius is numbered with the patriarchs who authorised them to teach this heresy - hardly successful then at keeping the Church from finding out his leanings…
“And with these we define that there shall
be expelled from the holy Church of God and anathematized Honorius
who was some time Pope of Old Rome, because of what we found written
by him to Sergius, that in all respects he followed his view and
confirmed his impious doctrines.”
I can’t see any wriggle room for you here. He taught from the ‘throne of Peter’, confirming the church in faith, that’s ex cathedra, that’s infallible. As it was a heresy that makes it infallible heresy.
newadvent.org/fathers/3813.htm