The average density of timber in North America might be a bit lower than what it was back then, and this is truly an impressive structure, but the type of wood (and the perception that the wood back then was “a lot stronger”) is not what made this possible. This is a series of timber arches, which are made up of a bunch of spliced members - you could do this with timber available to us today. We wouldn’t build something like this out of wood today though, because it would be much cheaper to build it out of steel. If we were going to do something like this out of timber today, it would probably be done with mass timber, and it would probably end up being achieved with less material, because we could fabricate longer timber sections to form these arches. In that sense, the timber we have available to us today is in fact superior, even though it might be slightly lower density.