Yeah if it knocked you on your ass like your making it seems chances are he would have been doing about 25km/h'ish around point of impact, no idea what he would have been doing prior to that though, depends on stopping distance and a whole lot of factors. You dont have to be going very fast to be able to lock up your wheels, depends on if you have ABS or not, how good the ABS is, suspension,tyres,brake type,brake material, road condition etc..
My car I've actually put to the test

When I first got it (99 skyline), the lowest speed I could lock up the wheels at on a nice day was about 75km/h, even then the ABS almost always stopped the lock up, felt like I was driving my foot through the footplate to achieve that. Now with my tyres only 0.3mm tred I can lock it up at about 45-50km/h without to much effort.
P.S im getting new tyres next week with the service for those that are worried