Yes, strangely, we haven't got a lot of experience with making very long extrusions of a quite new material. However, humanity has got a fair bit of experience with making extremely long cables. And with making load-bearing cables.
But NOT an extremely long load-bearing cable. Simply put: we've never successfully built a freestanding tethered structure more than about 5km high (tethers for observation balloons and some other structures). Gondola lifts and aerial tramways can extend this for a few kilometers only because the cables are supported by towers along the length and are not completely vertical throughout.
The leap from a cable car to a space elevator is NOT a trivial engineering challenge. It's similar to comparing an airplane and a space shuttle: despite their similarities, they have a completely different set of problems.
Surely long cables of a new material are going to be different in detail, but we have a lot of experience on similar, related problems in cable-making.
But not in multi-kilometer-long tether making. Just the massive distances involved makes this a very big technological leap.
Unquestionably a space elevator cable would be an enormous bunch of new challenges. I'm skeptical the thing can be done myself, much as I love the space elevator concept. But it seems quite clear the challenges in constructing such a thing are like the challenges of making a self-sustaining space station --- big, but roughly understood in the main, and imaginable that we could have workable detailed plans done within a generation.
Except that space elevators, unlike space stations, have the added problem of depending on an engineering process that we haven't actually developed yet (in addition to ALSO needing a self-sustaining space station to be in place before the elevator can even be built, but that's another matter).
If I recall my history there was a lot of talk among scientists about sustaining space stations as early as the 1930s, when manned space flight was still a theoretical at best. The discussion about space elevator tether materials is about as relevant as a 1930s discussion on what the combustion chamber of a moon rocket should be made of. Which is to say, it is probably NOT the biggest problem you need to solve, just the simplest.
I'm pretty sure at some point in human history we have evidence of cables being connected together.
But not tethers in a multi-kilometer long free standing structure, joined in such a way as to distribute the loads on both cables evenly without also causing a major shearing event while ALSO allowing vehicles to traverse the tether without damaging it.
That's not a thing that has ever been even REMOTELY attempted in the whole of human history. It's not that the art has been "lost" it's that it has to be totally reinvented to work in a completely different way before we can even attempt to TEST it.
