I've covered a lot of things from TERFs to the likes of Debi Pearl, but I saw this and honestly thought it was satire.
Look, diet culture sucks. Just eat as healthy as you can because you love yourself. However, that doesn't mean you can't treat yourself to a big--or small--slice of cake on occasion. If these fat acceptance people want others to stop judging them for what they put into their bodies, why are they doing it to others? No matter the size, a slice of cake should not be an object of guilt.
There are many reasons why someone would want a smaller slice of cake. Maybe they had cake earlier in the day or want some ice cream to go with it. They might have a smaller stomach, digestive issues, or simply not like cake. They might love cake but might not want to have as much of this particular cake. Even if they are on a diet, it's most likely none of your business. Most people who ask for a smaller slice aren't doing it to show off, but to make a personal decision about what they're putting in their bodies. Wow. What a concept.
Fat acceptance is a hypocritical movement in general. It promotes self-love, yet denounces positive change that results in weight loss as fat-shaming or body-shaming and promotes non-judgment, yet thinks people are bigoted due to their personal dietary choices. If you're going to be body-positive, support everyone no matter their size and support positive change.
It is not "fat-shaming" to ask for a smaller slice of cake, but there's always going to be someone who does ask this question to moralize. These people, however, have way more issues than just those that deal with cake. So have your cake--and eat it too.