A tad pedantic yes .. but ft/s is a velocity. Ft/s/s is an acceleration. I'm not sure where the 3.5 value comes from (a pedant would label it as -3.5) but it seems awfully small to me.
A bit of confusion here.
In this context, 3.5 ft/s is indeed a velocity - specifically it's the
change in longitudinal velocity required to activate the ELT, provided that's also accompanied by a minimum longitudinal acceleration of (minus) 2G.
At least that's the simplified version, there is actually a curve of G-vs-duration that defines the exact conditions required for activation.