The IDE that's written in Java most likely shares nothing with Java ecosystem parts they "hate" (I'm guessing all of the EJB, applets an general 90's Java enterprise stuff). Only irony here is you thinking it is.
The 90's Java was a thing to hate. Running fat complex app server with bunch of code-as-XML for configuration just to run simple API app. IDEs are not doing that (althought Eclipse was a bit of a monster of glued in parts)
> Did anyone in the mid to late nineties / early 2000s really discover Java and Java applets and thought: "Java applets is the tech that'll catch on, I'll invest in that" and not invest in Java itself?
...entirety of management it seems, it was the most popular hammer for the problem for quite some time in enterprise. Hell, we still have some less than decade old servers needing java applets for KVM