I would not call "solid lifter" engines defective.
No.
just cheaper to make
Mower engines are so unstressed and valve pressure is so low that cats urine would probably do for lubricating the cams
People pull stuff from web sites that is correct for the subject there and because they do not actually understand the fundamental science / engineering behind it think that it should be universally applicable
Perfect example is the valve seat recession that was going to cause every engine designed to run on leaded fuel to self destruct .
Did it happen , well very occasionally
why did every car not not fall apart ?
Well because the study was on air craft engines not car engines
So only applicable to engines running under lean burn conditions at 20,000+ feet
But every motor journalist jumped on the band waggon
Well ZZDP is exactly the same story
It protects from scuffing wear when the boundary lubrication of the oil breaks down
Can this happen on a mower engine ?
only if you fit double valve spring and rapid rise high lift cams and even then there would probably be enough oil splashing around to keep the cam lobes & followers happy.
This is of course provided you are running some thing a little heavier than sewing machine oil in the sump .