[philpem]

Not even "Not supposed to be" -- it can't be. $4489 decoded is a valid byte, but with one of the clock transitions 'blanked'.

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).