It’s a bit catch-22 - that Extras page relates to old style vanilla TinyMCE, not what I’m using/want to use. Nonetheless just for experimentation I rolled back to that at version 4.3.4-pl and hey-presto it removes the original issue of Rich Text fields being added to Description and Summary(introtext).
But… then it doesn’t include the <figure> <figcaption> feature and I can’t find the option to add it in a config file or from within MODX settings. Believe me I’ve looked.
I got round that a couple of years ago by using TimeMCE Wrapper but instantly got the same issue of Rich Text fields being added to Description and Summary(introtext) … as mentioned in my 1st post here - so catch-22 
With TinyMCE RTE I found this post really helpful: community.modx.com/t/external-configuration-of-tinymce-rich-text-editor/
… which in turn lead me here: tiny.cloud/docs-4x/plugins/image/#image_caption - which gave me the config for <figure><figcaption> etc. which is cool - but also the unwanted RT fields. So it’s catch-22 again 
Obv I could get round all of this by creating a new TV as plain text and simply using that for my <meta> data. But that would be defeating the object and going completely against my mantra of MODX being super-flexible, easy to use, a superior SEO CMS (actually paraphrasing the MODX team) and blah!!!
So I should be able to use Description and Summary(introtext) for <meta> without TinyMCE RTE - my chosen RTE plugin hijacking those fields >>> please !!!
Confused yet? I am!!! Do I need to hack the actual manager templates to circumvent this whole shebang?