Hearts are lopsided and don't look anything like what we now call a heart shape.

compare "inaccurate bloodpump shape":

https://nathanwpyle.threadless.com/designs/strange-planet-sp...

indeed, but I can imagine a combination of limited drawing skills and lack of reference combined with hundreds of years producing it as an outcome

I don’t know. Look at this photo: https://commons.wikimedia.org/wiki/File:Humhrt2.jpg

Except for that top part, the dark red part looks quite like a heart shape.

I thought it came from the shape of two mated swans with their heads together.