I take it, the mechanic is saying, drain the system to the extent that it will come out, which usually means what’s in the rad but not what’s left in the block below the level of the water pump. Then, fill the rad with pre-mixed coolant or a water/concentrate mixture. Not all the old coolant gets removed. The mechanic recommends compensating for this by doing the drain early. This will probably work and we did it like that on a friend’s Acura.
Having said that, I’ve been changing every 5 years, diluting the existing coolant in the block with water, running the engine until it heats up, then draining, and then adding enough concentrate to bring everything to 50/50. The vehicle has had this done twice and is approaching a third time at 15 years. No problems and it still has the original water pump. For the next change I might use the simple method and just drain and fill, knowing my coolant has been kept up to date for the last 15 years.
One other place where a simple drain and fill is convenient is with GM LS engines with no drain plug on the rad. You have to disconnect the lower rad hose, and it’s a PITA to have to reconnect the hose to run the engine with the flush water, then disconnect it again for the final drain.