When my first house was under construction in 2001, I created my own key from the builder’s key. When I finally moved in, my key would then disable the builder’s key. Curious how this work, I disassembled my lock and found that there were (iirc) 8 keys which would open all my neighbors houses, even after they had moved in and had disabled the builder’s key. Of course I rekeyed my house to prevent that vulnerability, but in theory it remains on all my neighbors. I also didn’t disclose the vulnerability.
I don't know about builder's keys from that era, but modern builder's keys aren't vulnerable to this problem. The builder's key uses a deeper cut on one or more pins, but the owner's key can and should be pretty unique. The only real requirement is that there be at least one cut to a high enough number to allow for a builder's key. There's a good video here: https://www.youtube.com/watch?v=GUCW4OnE6Mc
Maybe they used a master key system instead.