Hi Benjamin.
The answer to this is splitt in two.
I manage squarespace site who score poor on mobile but loads in less then 1 second. So this is more about how squarespace has setup its script and css loading.
As an example if I would use the developer account I would defer the two squarespace script til onload. But the common script wich is one of the two squarespace scripts is the script handeling image resizeing and placeing.
The sitebundle script already loads in the last section of html,
The thing here is cause of squarespace placement of css and scripts those who use a template that is not custom built will get this error. However it will not be critical for your site.
Google does not take this into account when they rank your site. True pagespeed is a factor but it is based on actual numbers, not this test.
This test is simply to help people make their site faster.
And as you say squarespace says that you should not mind this test. But that they do not adress it in a better form is bad for them.