I don’t know what you mean by “megafauna back then may have been pseudo-warm blooded”, partly because I don’t know what “pseudo-warm blooded” means. For the most part, an animal is either endothermic, or it isn’t (unless you’re talking about a few special cases like bumblebees, which use movement to raise their temperature sometimes, but are overall still ectothermic, but this is only really a thing in a few small invertebrates to my knowledge). Dinosaurs are believed to have all been endothermic, although like their modern bird descendants, they probably did have lower body temperatures on average than most mammals. But if you’re talking about past mammalian megafauna (or even present to some degree), the suggestion that they may have not been truly endothermic seems pretty baseless, as the range of average body temperatures in living mammals doesn’t really correlate with size very much (in that there are both examples of large and small animals with relatively high temperatures, and large and small with relatively low temperatures), really, adaptations to different climates and other physiological factors are much more important, and larger animals with high body temperatures require a more energy-rich diet to sustain the metabolic requirements, but to my knowledge that’s the main limitation. And all living mammals are true endotherms that generate body heat metabolically, even if there is some variance there.

Whales are true endotherms, and as far as we can tell, blue whales are actually the biggest animals in general have ever gotten (at least in terms of mass). A much bigger constraint on size is gravity (which is why aquatic animals have always grown the largest), followed by pulmonary efficiency (essentially, it takes special physiology to ensure that oxygenated blood can reach all the tissues it needs to in a big animal, and again, gravity is also a factor here when an animal is terrestrial, which is why it’s thought that the complex bird-like respiratory systems of dinosaurs may have played a role in them getting so big on land, in addition to the higher oxygen content of the atmosphere during the Mesozoic).

While terrestrial mammals never got as big as dinosaurs, they did once get a lot bigger than African elephants do today, such as the gigantic rhinoceros Indricotherium, and there’s absolutely no reason to believe any of these animals weren’t true endotherms (we obviously can’t know from fossils exactly how their body temperature stacked up to their smaller living relatives, but again, it’s not likely to have been nearly as important factor as many other things).

EDIT: also, as someone pointed out above, getting really big is actually in some respects harder for ectotherms, as it takes longer to warm a very large body in the sun, so they need to spend far more time basking the bigger they get. This is part of why cold-blooded giant dinosaurs were initially criticized as a theory, because a cold-blooded Tyrannosaurus, for instance, would literally waste all its time soaking up heat from the sun rather than hunting, and would just be far too plodding and sluggish to be an efficient apex predator.

EDIT 2: my bad, it seems that there are also some fish with a physiological means of generating some of their own heat, to the point of sometimes being referred to as “endothermic” informally, but they do so through different means than birds/dinosaurs or mammals do, and this also still doesn’t relate to any past land megafauna.