Did you actually scroll through all the way? Some emails send both plain text and HTML. I looked through mine and indeed saw text/html for most Amazon emails (including AWS). Some AWS emails like ACM cert renewals were text/plain. Though like someone else said here, you may have changed a setting in your account preferences. The default is still HTML.

    ------=_Part_4161549_398020174.1685704999675
    Content-Type: text/html; charset=utf-8
    Content-Transfer-Encoding: 7bit

I have zero html attachments.

(These emails are not marketing emails which I don't get, they all relate to orders and shipping)

They aren’t talking about HTML attachments. HTML email is typically sent as a multipart/alternative email with one text/plain component and one text/html component. An email with attachments is a multipart/mixed email.