http://amigadev.elowar.com/read/ADCD_2.1/Devices_Manual_guid...
It read full tracks at a time, but you can see from the doc there were 11/22 sectors with no inter sector gaps, but there are separate sectors of 512 bytes which are addressed in the file system structures.
If you decode $4489 via MFM then re-encode that byte, you'll get a 1-bit difference. This is why it works as a sync marker: even if you wrote that byte in the data area of the sector, it wouldn't encode the same way because of the missing clock :)
It's a common trick on radio systems too -- a short burst of data which can't be obtained through normal encoding processes (invalid FEC bits, flipped parity, etc).