The issue is that relays tend to require a lot more resources to operate than appviews and PDSes (though not necessarily as much as that blog post suggests; I recall posts of people running their own relays on RPi4s with NVMe drives), so it's common for alternative appviews to rely on Bluesky's relay instead of taking on the expense of spinning up a new one.
In any case, as that toot notes, the Bluesky outage was on a PDS level rather than a relay level. And thankfully it's much less expensive to run your own PDS; apparently those who do so weren't impacted.
The issue is that relays tend to require a lot more resources to operate than appviews and PDSes (though not necessarily as much as that blog post suggests; I recall posts of people running their own relays on RPi4s with NVMe drives), so it's common for alternative appviews to rely on Bluesky's relay instead of taking on the expense of spinning up a new one.
In any case, as that toot notes, the Bluesky outage was on a PDS level rather than a relay level. And thankfully it's much less expensive to run your own PDS; apparently those who do so weren't impacted.