It’s actually mathematically centered to its container but going to be off-center if you view it on smaller mobile device as the padding-left of the off-canvas menu will push all items to the right. Another issue if because the form isn’t responsive so it’s too big for smaller viewports.
Adding this CSS should make the form a bit responsive. (resizes the width to 100% of parent so it squishes)
nav#generate-slideout-menu form#mc4wp-form-2 {
width: 100%;
}