Someone likely would just docker/vm to partition that 80 cores into micro-service handle httpd proxy and app, db backend.
Those app/VM can easily be converted to app running pi modules.
20 pi modules likely have much better DDR, SSD, Network bandwidth. Probably scale from 2 pi to 200 pi as easily as typical vm setup - and it comes with GPUs for free for those need them.
20 pi modules of 8GBs is only 160GBs.
Ignoring that: Bigger nodes are better in all practical scenarios. With 768 GBs of RAM, this singular big server can likely keep in-memory a large collection of information (ex: all of English-Wikipedia likely fits inside of that RAM).
20x Rasp. Pi cannot access all of English-Wikipedia in RAM. This means that you can't index, you can't search, you can't analyze the pages. Even if you could: you'd need to have a collaborative external memory model, which is not easy to program.
80-cores with access to all 768GBs can do many, many more things than 20x 4-core Rasp. Pi working on only 32GB at a time.