Bad programming advice. jQuery uses best practices for many utility functions. Developers should not have to reinvent the wheel or code every basic call from scratch in every project. Don't act like using pre-built common frameworks is somehow a bad idea for ideal programming. They are reliable and faster, its one less issue to worry of what could be wrong.
Jquery's biggest feature was working around browser incompatibilities. This is much less vital now than it used to be.
If you aren't getting a lot of use out of the features it provides then you are better off without such a heavyweight dependency.
what is the alternative in this case?
There is no need for an alternative. You could go all vanilla with a polyfill for fetch.