So two problems...

1) I'm not sure what your problem with the reasonable rounding of 29 years ago to 3 decades is... but the one that comes across is "extra pedantry for no reason"

2) According to wikipedia the "first dos" attack was in 1996. There are other sources most of which attribute that 1996 panix attack as "one of the first" or "the first major" ddos attack. Before that there were other DoS attacks using udp and/or syn floods, and some of them likely involved several computers (and possibly people) working in coordination. Those several computers were probably not compromised machines that had malware responding to a cnc server, so the squishiness has to do in part with how exactly one defines DDoS - some definitions include a botnet requirement, others just need multiple computers working in coordination. It's claimed that Kevin Mitnick was targeting his prosecutor with syn floods in 1994 (over 30 years ago), but its not fully verified and the details are unknown from my research... likely though >1 computers were involved in that flood if it happened.

In the early 90s there were all sorts of fun and games where people would knock over IRC servers by triggering bugs/behaviors in a lot of connected clients. It's primitive but it seems to have a huge number of elements of DDoS. Similar for attacks on various telecomms infrastructure as the soviet union/eastern bloc fell apart in that time period.

Trying to put a hard "29 years ago" line in the sand is difficult to do... techniques evolve from previous ones and there are shared elements that make the line necessarily fuzzy.

So yeah... theres no reason to quibble about "three decades" since theres 35+ years of history around "things that look like DDoS attacks but don't fit a strict definition that requires botnets"