It's probably easier to only use 301 redirects, especially if the majority of deleted posts are not going to be restored and are deleted permanently in a few months.

That's what I thought they were saying at first, but if you read this line again, "... add a canonical URL of the category archive page to the individual posts..." they mean to add the tag to the posts, not the main page.

A canonical tag is used to signify that a page has the same or extremely similar content to another page. Here I don't know if that is the case, because the cornerstone page would have much more content than the individual post pages.

This is what Google says about the canonical tag:

"Must the content on a set of pages be similar to the content on the canonical version?

Yes. The rel="canonical" attribute should be used only to specify the preferred version of many pages with identical content (although minor differences, such as sort order, are okay)."

Well, with more pages you will have more opportunities to go after different keywords and offer new original content. As long as the content isn't duplicate or spammy, I can't imagine you would be hurt for simply expanding your website. Even if you added a bunch of internal links to your new pages, as long as your pages still look natural it shouldn't be a problem.

It's probably easier to only use 301 redirects, especially if the majority of deleted posts are not going to be restored and are deleted permanently in a few months.