A reference 3090 is longer by 69mm, wider by 29mm, and thicker by a slot than a PCIe A100. Though I think the comment you're replying to was talking about them

A reference 3090 is longer by 69mm, wider by 29mm, and thicker by a slot than a PCIe A100. Though I think the comment you're replying to was talking about them both using the same Nvidia GPU architecture, Ampere.