Update:
friendica uses the:
[text to be shown if image itself cannot be shown for any reason]
as altText as the:
"This is the mouseover text"
is considered the titel text of the image and is not displayed.

https://tupambae.org/display/ec054ce7-1465-aa01-e482-775401042197

There for it should be like this:
![basic avatar background of the actual tupambae.org server. The image is a gimped compilation of the friendica logo together with the CC published permafleur image and the copy left sign in the center of the image](https://pod.geraspora.de/uploads/images/thumb_medium_0923e4d988d129239c49.webp "basic avatar background of the actual tupambae.org server. The image is a gimped compilation of the friendica logo together with the CC published permafleur image and the copy left sign in the center of the image" )

doppelt gemoppelt hält besser!

btw
Actually the markdown site states:

Images
To add an image, add an exclamation mark (!), followed by alt text in brackets, and the path or URL to the image asset in parentheses. You can optionally add a title in quotation marks after the path or URL.

The example displayed shows the titel as hoover over. So somehow this in part is about the definition and or use of the the term altText. (?)
Like too say, mastodon considers the hoover over text a alternative text for the visually impaired while markdown considers altText as the alternative text when the image is not available.

Basic Syntax | Markdown Guide

The Markdown elements outlined in the original design document.

^{www.markdownguide.org}

quote @Michael Vogel

The first text is the text that screenreaders are using. So for accessibility reasons the first one has to be set. The second text is just decoration. It is not needed for accessibility.