- Didn't affect the atmosphere in any way. - Had all their buildings were recorded as being built by a people we know much about in specific timelines we know much about, which aligns with archeological artifacts. - Made sure none of their lithics stayed. - Used no fuel source. - Did no mining of any kind. - Never used anything but stone and paper machete for building (because even wood structures leave identifiable patterns for potentially thousands of years). - Only used that stone for a couple buildings, which they planned for the cultures that didn't exist to use intimately. - Had their bones dissolved without a trace.
All we have to do is remove any type of evidence-based science and anything is possible!
And, yes, I feel confident that with a few weeks, a rope, and a really good reason - almost any American could strap a horse to a buggy. It's not rocket science and countless humans have done it before.
It's not certain that much knowledge has been lost, although much of it is in "endangered" status of preservation. There's a kind of silk netting made from the hairlike tufts of a certain species of clams, only practiced by two people IIRC.
Some lost knowledge is being rediscovered. A well-known example is making Damascus steel; it's now so ordinary you can order it online.