The motivation/justification from the author why they believe removing lisp but adding Electron somehow sums up to being "lightweight"?

Maybe the author thought of the UX/baggage/legacy or something else when they thought about "lightweight", rather than how much memory/cpu cycles something is using? Not sure, but maybe there is a more charitable reading of it out there.

Probably none. Still I’m curious what is the authors understanding. Whether he actually thinks it is a lightweight solution or whether that’s kind of advertising phrase, like ‘blazingly fast’

I believe it's called a rhetorical question.

None, just another Electron hater.