You still haven't named the housebuilder which is even more important given your last post!
First of all, I do not believe injection dpc are a one-time fix.
From memory, it will need to be re done every 10-15 years. If you go down this route be sure to get an insurance-backed warranty as they often fail.
You are right, any surveyor worth his salt would quickly spot the absence of a traditional above ground dpc and would then be out with the moisture meter.
There is no substitute for doing the job properly.
If you do agree, it could open this bodge-job remedial to the many others in exactly the same position as you find yourself in.