ETT is but a guide, and depends on the head and seat tube angles. A degree change on either of these is 1 cm difference. So a bike with 73/73 head and seat tube angles and 55.5 cm TT may appear similar to a 72.5/74 bike with a TT of 56.5.
Reach and Stack are the true measurements to gauge fit. What is the geometry of your hack?
EDIT: assuming you are riding a 54cm Hack1, then the geometry is between small and medium for Defy. You will probably be fine on either. The 0.5 cm difference in TT length is due to the slightly steeper (72.5 degree) head tube angle of the Defy.
I’d personally recommend a Small for handling and lower front end, but a Medium will fit too. You will need fewer spacers, and a slightly shorter stem. Giant shops (and others) change stems for free if you ask nicely.