I've been looking into it some more and it seems to me now the creator of this comparison video has screwed up, because I don't see this stretching happening in other videos of the Switch version..
But in this (wrong) comparison the remaster was actually shown stretched, for example:
0:26 the towers and all circular forms throughout in the shot down on the castle.
1:24 the window in the door is not completely circular anymore. You can't see the original (circular) window in this comparison because they've been showing it off way too dark. But when you look it up you'll see the window wasn't squashed originally, it was a perfect circle like you would expect.
To me it seems really bad to send out a comparison video which messes up important stuff like aspect ratio and brightness.. It's like fake information : /