How do you measure poured concrete PSI?
As an experienced engineer, you are trying to beat a dead horse with details. The hammers are fun toys to play with, but I would not stake my life on the results.
It is difficult to get old concrete less than 3000 psi, but you have no idea of the minimum thickness unless you drill enough holes, when you eventually weaken the slab. - The minimum, not the average is what will control any failure.
There is no such meaningful thing as an average floor thickness. It could have been installed correctly or it could have be a "DIY week-end special" that was made trying to keep the cost to a minimum and fool the inspector for a shallow victory!
If you or someone else will be working on a vehicle held up by the lift and are injured you will get no satifiaction from a manufacturer's guarantee or an insurance company.
Just put in the lift according to instructions - it will be cheaper, easier and safer.