I think OP does have iCloud Notes on, just that they were looking at an Apple support note on how to send a text file/text file attachment to Notes vs wanting to send the text of an email to Notes.
But, they did find a workaround re: do a select all text and then send that to Notes.
The whole send to Notes share sheet does seem to be app dependent. But also make sense re: content/source type. For example, I can send this forum page to Notes, but it will take the link vs actual text (some sense: URL content is/can be dynamic; can always generate a PDF and send that to capture as-is at that time). Emails, loaded with images, tables, header information, so, lots of extra stuff that maybe makes more sense to share as a PDF (as noted by OP). Did a quick test of what the support notes says, and it still works: created an email with an attached .txt and .rtf file, both were sent to Notes as the full documents.