I know I'll probably get downvoted to hell for bucking the circlejerk but did you actually finish the article you posted? Because the author clearly says:

This is a technical point that does not detract from the letter’s message that any administration deal with Iran might not last beyond this presidency.

Also refer here for more info on the same issue. Interestingly, that same legal eagle seems to concur with the worthless GOP on this matter, saying:

[T]he President can waive most if not all sanctions against Iran for the remaining two years of his term if he is willing to make the requisite findings. If he does so, what are the implications for any nuclear deal with Iran? Answer: The deal will be tenuous.

And here's a legal article on the constitutionality of presidents unilaterally entering into nuclear arms deals. Or, if you're so inclined, you can acquire a Wikipedia-level knowledge on the subject of treaties here and on executive agreements here.

The general consensus seems that executive actions are expedient but not always constitutionally sound, i.e. if Congress really wants to they can sink them, i.e. what the letter said in the first place.

The general (and laughable and fantastic and childish) consensus on Reddit seems to be some variation of "use the Logan Act to purge Congress of all those pesky people who annoy us." Which is absurd considering that at its very worst the Senate's letter was nothing more than a poorly-executed commentary on constitutional law. Also because the Logan Act was basically designed to silence political opposition of the Quasi-War (along with the Alien and Sedition Acts).