Edit: the parent comment now seems to better reflect the below.
That article is only saying when you opt out there may be a loophole in the terms to allow OpenAI to train on the intermittent reasoning data anyways. If you don't opt out there is no ambiguity, all of the data can clearly be trained on.
So you have to opt out, it's just argued it's not clear from the terms that will also opt out of training on reasoning data or not.