For most light GA aircraft, where you would use slip to steepen a glide (eg in a glide situation posed by the original video), I would think you would not need to be particularly concerned by airspeed inaccuracies.
If the slip is being used to steepen a glide approach, so as to hit an aim point, surely you would not be trying to maintain a set glide speed, More likely, if high on approach, I suspect you would be pushing the stick forward rather than pulling back and not be worried much about what the ASI was saying, rather you would be trying to avoid overshooting the aim point.
And if you were pulling back - say to avoid undershooting the aim point - you probably would not be using sideslip.
In that situation ASI inaccuracies due position errors, would seem to me to be of small import.
my 2c