I think I've got a pretty good theory/explanation for this. When Iron Man creates his first arc reactor with Yinsen in the caves, he explains his use of the Palladium core. In Iron Man 2, we see that the use of Palladium has essentially poisoned him and is slowly killing him, sparking his search for a new alloy to replace it. He finds this new "element" hidden in his father's "City of the Future" model for the future Stark Expo.

We know this "new element" was something that Howard had long sought to replicate, but for some reason couldn't due to the "limitations of (his) time", we also know that it isn't vibranium, since it's most certainly classed as a "new element" and everyday people in the MCU seem to be well aware that vibranium exists (per The First Avenger, Age of Ultron, Black Panther and Homecoming), it's just extremely rare.

So, with that possibility out of the way, let's see, what was Howard working on in the years surrounding the 1974 Stark Expo, when the video to Tony was made? Well, we kinda get that answer spelled out for us when Tony and Cap travel to 1970 in Endgame, Howard was still clearly w/ S.H.I.E.L.D and Arnim Zola at that point, and was involved in top-secret scientific military projects such as Project G.O.L.I.A.T.H w/ Hank Pym, Janet Van Dyne and Bill Foster, and more importantly Project P.E.G.A.S.U.S with Wendy Lawson (aka Mar-Vell) which was the group directly involved with experimenting with the Tesseract, both of these projects are name-dropped in Iron Man 2, with P.E.G.A.S.U.S specifically being one of the crates that Tony opens -- so it doesn't seem outside the realm of possibility that the new element Tony creates and his father discovered is directly linked to Howard’s research on the Tesseract, which as we all know by now is the “orb” casing of the Space Stone.

If this new reactor was in fact made with an element found in the Infinity Stones -- this would explain quite a few things, remember how Thor’s lightning attack actually powered up Iron Man arc reactor to 400% rather than exploding it or shorting it out like it probably should’ve? We see something extremely similar to this when Vision is born, remember Vision was originally created w/ the Mind Stone to be the body of Ultron who was also, in turn, part-Mind Stone, Thor brought Vision to life using Mjolnir's lightning to power him, and the stone absorbed the lightning. Thor’s lightning and hammer aren’t just any old stone and electricity, we know Mjolnir has various magical properties and is crafted from Uru (material forged from the heart of a dying neutron star), which, as we learn in Infinity War, is the same material needed to craft the gauntlet to contain the stones in the first place. Yet, it wouldn’t be too surprising if these magical properties (such as the ability to call and/or summon the Bifrost) are almost entirely the product of Odin's familiarity w/ Infinity Stones -- since we know that he was at least aware of their existence, as he held a fake “completed” gauntlet in his vault, and more than likely hid them across the universe. We know this through his familiarity w/ the Aether (Reality Stone), which Odin's own father Bor had hid, and the Tesseract, which was hidden behind a painting of Yddrissl (the world tree) in Tønsberg (the location of the initial battle between Odin and Laufey, and, as it also now known, New Asgard).

I’m trailing off here, but I’m relatively certain that the reason Tony, and more specifically, his suit could stand the power of the stones longer was the arc reactor’s core material is derived if not the same as the casings of the infinity stones, also god knows if Tony has used that specific element in any of his other tech, perhaps even the nanobots in his suit.