Worthwhile to point out: The Cat5 cable required for gigabit Ethernet is merely twisted pairs with no insulation, which is pretty much a dumb basic cable (with 8 wires). That's why any cable can work in practice.

I don't know how possible it is to find a really bad cable (untwisted) and it might work on a short length anyway. (Your 1980s office cabling must have been 8 wires if you were able to get gigabit later, so it was far beyond basic phone wires or Cat1 from the time).

Sure, they will have been bundles of 4 pairs and I suppose we could say that is a matter of luck, it will have been installed from the outset in anticipation of networking - there's a period in the late 1980s when everybody is iterating on what will soon become 10baseT and the people in that building would have known all about it - but there's no reason back then to know 4 pairs will be an auspicious choice rather than 3 or 6.

So yes, those cables though they weren't Cat 5e because it didn't exist when they were manufactured, also were not basic phone cables, and I believe when the building was formally opened it had "ground breaking" 10Mbit Ethernet to every laboratory.