[Elsie]

I’m not sure why you are bringing up Innocent III.

Then it appears you cannot read and understand the question:

Is this statement infallible or heresy, or just mere ‘opinion’?

[piusv]

This is from the Lateran Council which is a general or ecumenical council. According to Catholic Encyclopedia:

All the arguments which go to prove the infallibility of the Church apply with their fullest force to the infallible authority of general councils in union with the pope. For conciliary decisions are the ripe fruit of the total life-energy of the teaching Church actuated and directed by the Holy Ghost.

So, yes, it is infallible. You can read more in the Catholic Encyclopedia online.

[piusv]

I think the way you broke it up into parts made it confusing for me.