She was saying that on Mastodon (that was the main activitypub platform she was comparing to) the choice of the instance can heavily influence your experience. If I don't remember wrong her main points were:

• There's a local timeline and a federated timeline, and even in the federated timeline you see your instance posts and the posts of the instances yours have federated with, not all posts
• A global search is not always the easiest thing to do, and previous attempts of project that would have facilitated it didn't received much appreciation from the community
• If your instance admin do choices you don't agree with (for example blocking another instance) the only way to interact with that other instance is to move yourself
• Moving from an instance to another means loosing your posts and replies, that would stay on the original instance

She was not saying that this approach is wrong, in fact many people on Mastodon like this more community-focused and less-global approach, just that it isn't what they wanted for Bluesky