This question already has an answer here:
Anyone know how I can enable the WYSIWYG editor for the textarea summary field in D8? I have a "body" field on my news content type set to "Text (formatted, long, with summary)" but only the main textarea gets the WYSIWYG editor while the summary field is just a standard textarea.
The summary field does support rich text, presumably using the same text format as the main textarea, but it's not clear since CKEditor is not there.
Note there's a similar question asked but it's from 2011: How to enable WYSIWYG in a text field summary?