They're still humanoids, so I always figured they were warm-blooded.
Being bipedal and sapient doesn't make one warm blooded. There are billions of stars in our own Universe. It's possible other bipedal species live on other planets, and the likelihood they're all warm blooded because they're bipedal makes no sense. It's like saying all quadrupeds should be cold blooded. It's more complicated than that.
There's a precedent in our own planet's history for warm blooded reptiles and reptile-like mammals. Many forms of Dinosaur fit into this category, etc. It's possible Argonians are warm blooded, but I wouldn't say it's because they walk on two legs.
Biology in Tamriel is different anyway. Things like evolution and genetics work differently. In this Universe gods definitely exist and all the sapient humanoid races appeared almost fully formed shortly after the world was formed as the descendants of greater spirits who helped form the bones of the world. The Argonians are closely related to the Hist whereas Men and Elves descended from Aedra.
Like the Khajiit, they are descendants of the beast-people.
And people sound and probably are warm-blooded. Maybe they've kept the warm-blood.
"Beast people" is a highly subjective term. Until recent history Orcs were considered beast people. Argonians and some nationalist Dunmer consider man races to be apes and not human.
I'd be loathe to put too much stock into a biased and loaded term like this.