There are a lot of things people might mean by claiming that something "is just JavaScript," but one possible meaning is that the source code you write can run in the browser without any build whatsoever. For React, that's true with the exception of JSX, which is a very simple and optional syntax transform. (Of course in practice you'll probably want to do module bundling too, but browsers could technically load your ES modules directly from static file storage.
For Marko, that doesn't seem to be the case, but it also doesn't really make sense given the problems that Marko is trying to solve.
Another thing people might mean by "it's just JavaScript" is a much more subjective notion about how similar the overal syntax, control flow, etc. feels to whatever previous JavaScript experience the person has. This meaning is a lot harder to pin down, and in most cases reasonable people could disagree on what is and isn't "just JavaScript" according to this meaning. That said, I would tend to agree that React's templating uses normal JavaScript control flow and composition primitives more so than Marko.
For Marko, that doesn't seem to be the case, but it also doesn't really make sense given the problems that Marko is trying to solve.
Another thing people might mean by "it's just JavaScript" is a much more subjective notion about how similar the overal syntax, control flow, etc. feels to whatever previous JavaScript experience the person has. This meaning is a lot harder to pin down, and in most cases reasonable people could disagree on what is and isn't "just JavaScript" according to this meaning. That said, I would tend to agree that React's templating uses normal JavaScript control flow and composition primitives more so than Marko.