DNA sequencing wasn’t an economically feasible option 20 years ago

OK 20 years maybe it wasn't worth the cost, but by 2011 it was only thousands of dollars.

https://www.genome.gov/about-genomics/fact-sheets/DNA-Sequen...

https://en.wikipedia.org/wiki/DNA_sequencer#Comparison

Well you’re not addressing the GPs concerns. The DNA could be severely damaged.

Ouch, it is not uncommon to freeze samples for a long time, especially rare ones like this. You're free to peruse the literature on how this is an accepted practice.

So I did. Searched and read about it, found out the viability and recovery of cryopreserved PBMC has a clear downward slope. Samples are good for years, not decades.

This depends on what you want to do. The DNA can certainly be sequenced. They might even be viable if you’re nice to them.

What I mean to say is this is an accepted practice in many labs, including ones I have worked in. In my experience, these were old tumor cells stored in LN2. Sources include: https://link.springer.com/article/10.1007/BF01131376

https://pubmed.ncbi.nlm.nih.gov/9336042/

https://pubmed.ncbi.nlm.nih.gov/27792414/

If they did analysis using tools and methods that weren't available / affordable 20 years ago, then preserving the samples for that long might have been wise.