I don't think this is the most robust approach to fixing this issue. If I am understanding the problem correctly, the issue is caused because the Enhanced Responsive Images plugin modifies the sizes attribute using the wp_content_img_tag filter, since we're not getting all of the information we need to filter the wp_calculate_image_sizes() value directly.

However, when Modern Image Formats applies webp_uploads_wrap_image_in_picture() it adds sizes attributes to the source elements by calling wp_calculate_image_sizes() directly, rather than using the actual sizes attribute that is on the img element that it is wrapping.

I think that a better solution would be to update webp_uploads_wrap_image_in_picture() to try to copy the sizes attribute from the img that it's wrapping, if available, rather than using wp_calculate_image_sizes() directly. This would ensure that any modifications that were made to the sizes attribute, regardless of what filter was used, will be applied to the source elements as well.

Alternately, we need to make sure the enhanced sizes attribute ends up getting applied via the wp_calculate_image_sizes filter so that any other code that calls wp_calculate_image_sizes() directly will have the optimizations applied as well.

As a side note: when encountering compatibility issues like this one, we should be skeptical about modifying filter order as a way of fixing the problem, since we can't control the filter order of all third party code that might modify the HTML markup that our plugins affect.

Looking good! Almost there.

Just a couple nits, but pre-approving as LGTM.

Excellent!