It seems to mainly be true - it clearly does not have final sign-off so nothing is completely off the table (yes, they could ask for more) but doesn't seem to happen that much.
I wouldn't assume that the security/background/criminality checks haven't started - again, may just be awaiting final confirmation or sign off. I don't know for sure but doubt that they would get to this stage and write pre-arrival if there were any 'heightened' checks flagged in the files.
Some seem to get decision made and even PPR fairly shortly after pre-arrival, sometimes quite quickly, although there is variation and longer cases. (More variation right now - covid and travel restrictions).