Is it okay to post copy/pasted answers from other websites?
I guess it's not.
The link to website can be posted in comments instead.
There is a section on plagiarism in the Help Center:
Plagiarism - posting the work of others with no indication that it is not your own - is frowned on by our community, and may result in your answer being down-voted or deleted.
The answer you link to is an egregious example of plagiarism and should be removed. It contains no original content and no effort to adapt or contextualise the copied material.