xkcd

From what I can find, an average sweater uses between 3-6000 feet of yarn [1]. Apparently an 'ideal' altitude for for skydiving is 14000 feet [2]. That puts you around the same altitude as high altitude aerostat radar systems[3]. It would have to be a fairly voluminous sweater, but it's probably just about within the realms of possibility. It's chilly at that altitude, so maybe it was made with several layers for extra warmth?

I'm no wool-ologist, and even less of a jumper out of planes, but these were the sources I used to get my figures:

[1] https://www.blackeryarns.co.uk/advice-information/wool-ball-lengths/ [2] https://friendlyskydiver.com/skydiving-altitudes/ [3] https://en.wikipedia.org/wiki/Tethered_Aerostat_Radar_System

The comic lists tall structures existing in this moment. Aside from not starting at ground level, as an experiment, the space tether does not persist until today.

The Great Pyramid in Giza, as it is really old?

All of needs is a qualifier. "Permanent" or some appropriate synonym would work. Also, "structure" is probably technically true, but not colloquially, common-usage true.

But: it's a comic. XKCD's main failing is that it's mostly a serious, science-based unless it's being outright silly, which triggers the pendant in us. Or, in some of us. My defense mechanism is that Randall is far, far more educated about these things, and I immediately fell stupid for being pedantic; like how you might feel when you find yourself criticizing the phrasing of a sentance in "On the Electrodynamics of Moving Bodies".