While I don't have any screenwriters to cite, the answer is no.
Here is the reason: Manga is written and published as a serial, the superstructure of the story is highly subject to change as the serialization progresses - based, of course, on reader response and the need to stay serialized, working closely with an editor to refine ideas to appeal to a broader readership. This is more like a comic book. A screenplay may see revisions, but they are not changed based on reader response. If something is very poorly received in test screenings, the production company may cut, rewrite, or reshoot some specific scenes even in the postproduction phase if it looks like they will lose money as a result of the inclusion of the original content but this is not generally a compatible comparison to a serial.
Writing a graphic novel is probably more similar to writing a screenplay. A graphic novel being a standalone literary composition visualized and set within either A) a pre-existing world/continuum, or B) independent of any pre-existing world or continuum but can at times contain known or unknown characters within a known world and may not always follow the original world's rules or patterns.
If you ask "is writing manga more similar to storyboarding a screenplay?" then the answer is yes, it is perhaps similar to storyboarding a screenplay, but probably no more so than it is for a well written comic book or graphic novel - simply that the mangaka generally does all the heavy lifting (with the help of assistants of course) where as a "comic book" or even sometimes a graphic novel has a writer, and various tiers of artists and typographers in addition to the editor, the mangaka (with the exception of collaborations or collectives like CLAMP) is both the writer and principle artist.