How much does Google penalize search rankings if one takes their blog from WordPress/Square/Webflow/Hugo/etc to another platform - while keeping the same article text body and keeping the same article URLs?
Google doesn't care about the serverside engine only indirectly it affects your resulting page. And this you can check via: https://developers.google.com/speed/pagespeed/insights/
Indirectly all of them have an effect on the final page. But with every of these, you can get good SEO. None of them is particularly bad - you should probably care about a particular template and set of modules pluggins you use on them
Looking into the Google guidelines I was not able to find anything closely related to your concerns. The only thing that popped up was the duplicate content and possibly content scraping issues that you may face and from where you may trigger a penalty if and only if you are not careful and planning to keep both website up.
However there are legitimate reasons why you may want to move the content across and if you do that follow these recommended steps assuming you want to change hostname but keeping blog posts paths or urls the same. If you want to only change the hosting or CMS platform using the same hostname follow the move with no URL changes instructions.
If you want to keep both websites up use a technique called content syndication following Google duplicate content guidelines here (see “syndicate carefully” advise)