That sounds a bit urban legendy, although it's difficult to prove a negative, and necessarily quite difficult to source. Indeed, I suppose I'm being too absolute, because I'm sure someone along the way may have decided that demons or fairies helped with the old ruins, but the idea that "Medieval Europe" in any consistent/universal manner saw no explanation for "seamless floors and pillars" short of magic smacks too much of the "ignorant Dark Agers" trope for me to lend it any credence unless someone can substantiate it with medieval evidence.

Roman architecture is impressive. Concrete helps make it so, and sometimes the seamless construction is part of the impressive aspect of it -- the dome of the Pantheon comes immediately to mind -- but your quoted OP seems to be committing ridiculous hyperbole at best.

Medieval people knew how to make various forms of concrete (though not the Roman concrete that could set underwater, which even the Romans may not have fully understood how to make once they exhausted the lime mine whose unique impurities made the concrete it produced exceptional). But you find other forma of concrete theoughout the middle ages. For example, the floors of the timber longhouses at Lyminge, England (7th century) were made from concrete, and the roof vaults of Salisbury cathedral (13th century England) were made from hardened lime. Much of technology wasn't forgotten, was used for various projects, and changed and developed during the thousand years of the middle ages.

But the question of magic is more complicated. Modern people like us use the word magic to talk about things we can't understand (I don't know how he did it, it must have been magic!). But medieval magic is often used to make a bigger, better result from skilled technological processes: a sword that doesn't grow dull, a smith who can make repairs faster than a normal man. I would not be surprised - and actually, I would rather suspect - that a medieval audience, seeing a wonderfully executed engineering project would simultaneously recognize that it was produced by skilled craftsmen and suspect that those craftsman probably had some special magic that allowed them to take their craft to a level beyond the ordinary.

Kind of a slightly bull-shitty assumption actually, as medieval Romans would routinely strip ancient buildings of their stone/marble/statuary and bake them in ovens so as to get mortar for their constructions. Impressive buildings such as the Torre delle Milizie weren't the work of magic and they certainly didn't build themselves...

I think saying: "Medieval Europe thought the ancient Romans were magic" is very inaccurate. There was no homogeneous group of people with a singular idea about a subject. Not for roman engineering, probably not for anything, anytime, anywhere.

I wouldn't doubt there were people who thought some of the major roman works were the product of some supernatural forces (I mean, even today with all our learning there are people who think the pyramids were built by aliens!), but I should add that most roman ruins were viewed by a very limited number of people. Mostly the rural people that lived nearby. So it's possible that they created legends to explain the most impressive of them.

Most learned and more well traveled people would probably know that the romans were people just like them, who nevertheless built better in the past that they could then. Not because they had magic or anything, they just had more knowledge.