The article confuses mixture quantity with mixture strength. Part throttle gives less mixture not necessarily a leaner mixture. Peak cylinder pressure should be slightly before TDC. 11° BTDC comes to mind but it has been awhile since I read up on all this (not on the internet, in technical manuals). Idle and part throttle mixtures do indeed take longer to burn and may benefit from more spark advance. How much more primarily depends on combustion chamber and port design.
Increasing part-throttle timing advance can greatly increase fuel economy and improve drivability.
Increasing idle timing advance can be problematic, particularly with manifold vacuum. Idle timing advance much above 15° may lead to an uneven, hunting idle. Using manifold vacuum to advance idle timing means the ignition timing retards suddenly and significantly when the throttle is opened. This often creates a huge flat spot that can be very difficult to eliminate. Using manifold vacuum also gives maximum vacuum advance at idle which may mean total vacuum advance may need to be limited for a satisfactory idle and be less than is needed for best part-throttle operation.
Contrast this with ported vacuum advance which gives a nice increase in advance when the throttle opens. This really improves drivability.
I have worked on cars from the 20s on up. I was at ground zero dealing with drivability issues when emission controls were introduced. I'm sorry but the author is mistaken, port vacuum advance was the norm prior to emission controls. It was used because it gives an even, steady idle, good drivability and good part-throttle economy.
Now back to sidedrafts and part-throttle vacuum. Only a few side draft carbs had a ported vacuum outlet, very few. There are good reasons for this. Larger throttle bores mean that port vacuum signals are smaller in magnitude and exist for less time as the throttle opens. Sidedrafts are also frequently set up with a single carb bore feeding a single cylinder, this gives a pulsing vacuum signal not suitable for vacuum timing control. You can fit vacuum "dampers" inline, or mix the signal from multiple bores but it all starts to get rather complicated for limited gains.
This is where electronically controlled timing advance really shines as you can use rpm, throttle position, manifold pressure and knock sensors to deliver the ideal timing to suit a huge variety of conditions.
PS: I have greatly simplified things as all sorts of other factors come into play with ignition timing: bore/stroke ratio, piston area and design, etc.
