I am not going to speculate.
> The GDPR actually defines personal data as "any information relating to an identified or identifiable natural person (‘data subject’)...", which is significantly different to what you wrote.
I can't speculate on what you think qualifies as "significantly different". I copied and pasted my definition from the ICO's website, which I also linked to. You can disagree with them, but I suspect strongly, even in the current Brexit climate, that the EU courts would agree with the ICO's interpretation over yours.
You might find the part that follows the "..." useful though. It's in Article 4 § 1, if you're unaware.
> It seems to be widely understood, including acknowledgement in various statements by EU officials, that these provisions were aimed squarely at businesses like social networks to avoid them locking users in by holding the user's data hostage. That seems to be exactly the scenario we're talking about here.
That may be part of your confusion. "We" weren't talking about anything to do with social networks, but whether a GDPR regulator is going to levy a heavy fine to Trello for this behaviour.