I just want to preface this by saying I am not coming into this with hostility, but because I'm curious about this and would like to understand a bit more about where you're coming from (if this is something you believe and you're not just explaining what the meme says?)
I agree with and understand the first statement in the meme, that "sex" is how we describe various biological differences (e.g. "sexual dimorphism") and how we categorise people medically (afab, amab). I kind of viewed sex and gender as two different things, one medical and one personal/cultural. Neither binary, just to be clear, I understand that biology isn't a binary thing and people are often born with a mix of sex characteristics, which is part of why I thought "assigned gender at birth" terminology was quite good as it just describes what a person was assigned at birth, rather than conflating gender and sex and asserting that a person is a particular sex, and that it can be useful to have afab and amab as medical shorthand for things to look out for when dealing with surgeries or medication or diagnoses (e.g. amab people aren't likely to be pregnant for instance, I dont know if that's true but as an example).
But my understanding of what you're saying is that sex and gender are much more closely linked, and that neither are biological/medical in the way I described above - in fact that gender informs sex, and is in turn informed and enforced by culture and power structures.
Is that an accurate understanding of what you're saying? Or is that much too simple? I'm not a scholar, obviously, but I am interested in making sure I understand how people interact with and understand their own bodies and identities etc as I have trans and non-binary friends