> Both errors will be mapped into the same Failed to fetch JavaScript error, so we can’t rely on the error type, but we can perform a timing attack. Local networks are fast, so the valid mDNS hostname registered in the network will be resolved in a reasonable time frame, which is significantly faster than the default connection timeout. In the example above, the difference is four milliseconds for a valid address versus five seconds for an invalid one.

This isn't really a "timing attack." A timing attack implies exploitation of an unintended behavior by executing something at some specific time. This just sounds like a workaround for the fact that the Javascript API doesn't expose a way to detect a hostname resolution error, which really should exist.