To get a classifier that worked in 2019, you had to train a model. Either from scratch or from a starting point.
GPT2 was absolutely unusable as a classifier. Using GPT3 as a classifier cost a few order of magnitude more than what this thing is priced at, and the context window was a few thousand tokens.
> These kinds of zero-shot classifiers were already developed and used in-house for many years
This is like Google's favorite coping mechanism for falling behind at AI. "We had everything inhouse for several years, we didn't release it for $reasons."
> anyone who could use an LLM proper could build layers around it to fulfill any classification task like this
You missed the part where it costs more than two orders of magnitude lower :)
I'm not claiming there are major architectural innovations, but that's not the point. Once you prove there's a market, there's a cambrian explosion of innovations.