u/danjl was originally asking about "this class of drugs", which is more reasonably interpreted as referring to personalized mRNA vaccines rather than the much broader concept of (personalized or not) immunotherapy.

The comment is wrong because it's stupid. Just because we've known that personalized immunotherapy was a promising idea for a long time, and have been trying it with the limited tools we had (CAR T being the poster child, and while an incredible advancement for blood cancers and many other diseases, it is notoriously dangerous and expensive), does not mean we had the technology to accomplish it. Today's results are only possible because of recent advancements in multiple fields, and to respond with "they've tried immunotherapy for decades" is profoundly ignorant. You may as well say, "scientists have been trying to cure cancer for years."