silverpillRequirements in that section are meant for those who want maximum compatibility. For example, JPEGs and images larger than 256 KB are supported by Pleroma and others, but Mastodon doesn't allow them.
Non-square emojis are a bit more complicated. They are not rejected by most servers, but might not be displayed properly by clients.
I just visited your instance and made a screenshot of how emoji is rendered by web UI (attached). The emoji is present, but I can't tell what is depicted there. Some clients enlarge custom emojis on hover, but not Mastodon, so I have to open the image in a new browser tab, or zoom in the page. This is no ideal, hence the recommendation in the FEP.
All GNU social JP content and data are available under the