I doubt there will be many pro-RIAA perspectives here so as someone familiar with their leadership and goals, I might as well add some perspective.
Imagine yourself stumbling upon some incredible technology breakthrough. Let's say, it's some magical script you wrote that automatically improves the efficiency of any container by 70%+. You've worked for years on this specific problem, went broke a couple of times trying to figure it out, and finally struck gold. You form a company, issue that IP to the company, and start getting rich.
Then one day, literally Google comes along and posts your script to some website they own, without your permission. They did this illegally, without your permission, by using this other neat script called gofyourself-dl. You have a problem now. Every is improving their containers or whatever for free. Your company loses revenue every day because why would anyone pay you for tech they can get for free from Google?
What can you do? Sue Google? Good luck with that! No, you would complain to a regulator, which is the only real power (sort of) against that kind of money. You would try and convince that regulator to stop Google from stealing your IP and getting rich from it because it is YOUR IP and YOU should get rich from it. In the meantime, since gofyourself-dl is the way people are stealing your IP, you naturally try to get that taken down because IT IS BEING USED TO STEAL YOUR IP.
Stealing IP is exactly what YouTube is - and has been - doing, for years. There is this weird idea that the major record labels (who fund the RIAA) have unlimited money and are just as evil as blah blah blah, in reality the entire music economy is a minuscule fraction of Google's market cap, power, size, influence, everything, and large tech companies have no real incentive to deal with them in the standard cases because why would an elephant concern itself with an ant.
It sucks to do this to an open source repo, but everything they are saying in this memo is true - this technology is obviously being used to STEAL IP. Its primary focus is the THEFT OF IP. DMCA is a shitty blunt instrument that no one likes, don't for a second think the RIAA is thrilled with that mechanism, but what else are they supposed to do? What would you do if it was someone stealing your proprietary code and posting everywhere after having stolen it from you?