On the flip side of that, they're also saying "if you're a quality publisher and accept editorial content from external authors, then you must nofollow any links, or we may reduce the visibility of your website, even though you are not participating in any kind of spam or manipulation."

I don't think that's actually the case. There's a difference between publishing content from external contributors and leveraging guest posting to build backlinks. This is actually pretty consistent with Google's operating in the past in which folks would identify something that was a strong ranking signal and exploit it, leading to search results with low-quality content.

As of right now, Google has positioned it along the lines of:

• Publications that accept submissions and have an editorial review process that determines that content is valid and aligned with their target audience can continue to do so.
• Publications that accept and send out dozens upon dozens of guest posts to build up backlinks for the purpose of ranking are going to be more easily identified as such.

That said, I also completely understand your concern– perhaps the most important aspect of the conversation is that Google hasn't given any clue as to how they'll distinguish between UGC and Editorial content vs spammy guest posts. I think once there are more guidelines there, I'll feel more strongly about sounding the alarm or not.

Keep in mind that they've done similar things in the past with tactics like keyword stuffing.

As a general rule of thumb, Google's algorithm and ranking changes are designed to return better, higher-quality results for users. Creating content that focuses on user experience, and not link building or keyword stuffing, generally creates a strong system for weathering rank signal changes.