If you mean that the line doesn't extend all the way to 2Hz, you're looking at the wrong line
The extra-squiggly line that stops short of 10 is your file, the smoothed line that goes down to 2 is mine. How far down to the sub Hz it reads is just an artifact of REW, that I compensate for in the case of the 1024 tap filter by adding zeros after the end (and I overcompensated)
A short impulse won't crop out your lows, it will just not be able to control their response in any resolution. e.g. that's why the graph is smoothed out.
I did the cropping in Audacity, used the dB view of the vertical axis to see the low level content of the impulse more clearly (after setting it to go down to -145dB).
Added lead in and lead out via "Fade in" and "Fade out" commands.