That paragraph is talking about leaning for cruise performance / efficiency. The other commenters appear to be talking about leaning during climb.
In a naturally aspirated engine as you climb the MP drops and at some point the mixture at full rich will be too rich and engine performance will suffer and you need to lean while climbing to maintain proper engine power.
In a turbocharged engine you can keep the MP high all the way up to the critical altitude so you should keep the mixture full rich during the climb. Due to how MSFS handles turbochargers however this isn’t the case with the turbo arrow and you need to manually keep pulling the mixture back even though the MP is still high.
I have been using this mod which corrects this behaviour so even though the position of the mixture lever in the virtual cockpit (in the sim) is wrong, at least it is correct in real life on my controller and I don’t have to keep pulling it back in the climb:
I don’t know if it will help but I thought I would use your example of leaning in the cruise to explain the situation better… Say you are at 6,000ft cruising with the MP at 26" and prop at 2500rpm (55% power), and you want to want to lean to peak EGT for best economy (as per the POH). You would find a specific spot where your mixture lever should be to get peak EGT at that altitude and power setting.
Now climb up to 12,000ft and set up the same cruise power (26" and 2500rpm). The exact same spot on your mixture lever SHOULD give you peak EGT, but in MSFS it doesn’t. Since the turbocharged engines aren’t simulated correctly you will need to pull the lever back much further to get peak EGT. With the add-on I linked above your mixture lever should be in the same spot as it was at 6,000ft but if you look at the lever in the sim the lever will show as much further back.