I agree. I have seen websites that say the same thing. Maybe diesel engines require more warm-up time, I don't know.
i usually figure its warmed up when i see the temp gauge move even a little. that temp reading comes straight from the coolant temp(at least with most cars). so if your engine is hot enough that your coolant is warming up, its warm enough inside the block.
that heat has to go from the combustion chamber, through the metal in the block, then into the coolant. well, the temp is usually taken at one spot in the cooling system. so that heat also has to heat up all the coolant, until it reaches that temp gauge.
id guess the very bottom of your temperature gauge is probably at about 100 degrees. for that coolant to be able to move that gauge, its gotta be hotter than the minimum on your temp gauge.
it doesnt have to be HOT, just warm enough for the metals to expand a bit, and gaps to close. oil also loses some viscosity when it is warmed. if your doing 5,000RPM's with a cold engine, that thick oil does not get into the small gaps it needs to in order to lubricate the moving parts. this causes scratching of metal on metal, and heat buildup. this obviously causes problems in the long run, sometimes the short run