Yup, the “zero” frame counts as one of the frames where you can link!
Lots of people here seem to be confusing hitstun with blockstun.
You can’t really figure out links using frame advantage as reference. Remember, frame advantage is something calculated from how much recovery a move has vs. how much blockstun it gives. The formula for this would be:
F = B - R
where:
F = frame advantage
B = blockstun
R = recovery
For example a move with 7 frames of recovery but only 2 frames of blockstun is has -5 frame advantage (i.e. is therefore unsafe).
Links in combos however are based on hitstun, not blockstun and hitstun has nothign to do with frame advantage. You only calculate frame advantage on block, not on hit.
To calculate whether or not something will link, you need a different formula.
H - (R + S) = Fl
where:
H = hitsun
R = recovery of the first move
S= startup of the second move (before the active frames)
Fl = frames to link.
An example of a 1 frame link then would be having a move with 8 frames of hitstun, 4 frames of recovery being linked to a move 3 frames of startup. Written out in formula, this would be:
8 - (4 + 3) = 1
As you can see, this has nothing to do with frame advantage and blockstun (since the former is a value calculated from the latter).
TL:DR version: You need to use hitstun and not frame advantage to calculate for combos.
More on this can be found here.