Maybe Elrond thought at that time the foundations were indeed made with the power of the One Ring, but that they later found out that this was not exactly the case. It's a possibility they later found out Sauron already around the year 1000 SA began building Barad-dûr, hence the reference in the Tale of Years.
I guess Sauron began building his tower and later reinforced the fundations with the power of the Ring.
Another possibility is, that Sauron was at the same moment studying the arts of ring-making whilst building Barad-dûr and already used something of the lore he learned in the creation of its foundations.
A last thought occurred to me while writing all this: as the power of the Ring was in essence Sauron's power concentrated in an object, maybe we should view also Elrond's reference thus: Sauron's concentrated power in an object. As Elrond both could feel the power of the Ring and the power in Barad-dûr's foundations, I think they were the same to him: Sauron's concentrated evil power.
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