I don't know where that book gets its info from or anything but some of it is true;
The S.m.E. round is the round designed to be fired by the MG's Glider, it's an AP round firing a completely similar projectile to the S.m.K. except optimized for MG's. The S.m.E. round wasn't meant to be fired through the K98k as the K98k was optimized for the s.S. round, so obviuosly firing the S.m.E through the K98k you wont get the best results.
However also note that Snipers wouldn't want to switch between different ammo types as their rifle scopes were designed calibrated for the s.S. Patr.. German Sniper scopes had built in range adjustment, with range adjusted in 50m increments, and this was based on the ballistics of the s.S. Patr.. Now firing s.S. V-patr. wasn't a problem as one just had to keep in mind that the trajectory was flatter, however using MG ammunition such as the S.m.E. round with different projectiles powder charges would affect accuracy as both the rifle scope were designed calibrated for an entirely different round projectile.
Next there was no "Specialized" Sniper ammo, the German Snipers however sometimes "borrowed" high power s.S. V-patr. from the LW to gain an extra 150m of effective range. So when the quote refers to specialized ammo for snipers it refers to the production of s.S. Patr. for Snipers only in 45 which otherwise had ceased.
And as for the the quality of German rounds (s.S., S.m.E., S.m.K. etc etc) it's excellent (I have these rounds myself, they're all better made than equal time period US British ammo). Esp. German tracers are very well made.
The German standard sS (schweres Spitzgeschoß/heavy pointed bullet) ball bullet was 35.3 mm long (1.389 in) long, boat-tailed, and very well made.[2] It was lead filled, had a gilding-metal-plated jacket, and weighed about 12.8 grams (197 grains). It offered the best aerodynamic efficiency and ballistic performance of all standard rifle balls used in World War II.
German tracer bullets were the best put out by any country, beautifully streamlined and with excellent ballistics. German armor piercers were also very good, being very stable and accurate at long ranges.
The most accurate rifle round of WW2 was the s.S. Patr., and it was therefore also prefered by Snipers. However it is true that this round as in short supply by mid 1944, and this did prompt an increase in production of it for use by Snipers only.
I recommend reading Peter Senich's book on the German sniper which notes this as-well.