The comparison of i-net PDFC is primarily focused on the print output of documents. As a result only elements with a visual appearance are compared in the current version. Since alt text is an attribute of the image and not an annotation element, it is not yet compared.
We are currently working on an improved support for PDF/UA documents which will include alt texts as well. This feature is slated for version 25.10 which will be released in October 2025.