That link only describes what the author calls an "impact slug". The 'impact slug' will provide different information, depending on the chambering and particular rifle in question. Sometimes, it may only provide throat diameter; and sometimes it may also show some rifling and bore diameter; but it will never show anything in the middle of the bore. That method is not as common as actually driving a lead slug all the way through the bore.
Although I agree that it is theoretically more important to know the bore size of the chamber end, it can be just as important to know if there are any constrictions or loose spots in the bore.
Only by slugging the full bore, in addition to the throat, will you know what the barrel really has to offer.
I have a .380 Auto pistol that has a .358"x359" bore diameter (it's oblong). However, it has a 1" long loose spot in the middle of the bore, large enough for a .358" slug to 'fall through' the loose spot. Knowing that I have the loose spot forces me to load only hollow base bullets, to help avoid leading. (I don't know just how large the diameter is at the loose spot, I just know that it's bigger than .358"x.359", but hollow base bullets eliminate leading.)
I also have a .30-40 Krag that I recently slugged in three different ways (using soft lead .315" wadcutters):
The throat measured what I view as an acceptable .309" diameter.
However, the bore has both loose, and tight spots. I don't know how big, or small they are... but now I know they're there. I could feel them, as I pushed the slugs through the bore.
And... the muzzle/crown is severely worn. In less than 0.5", the bore tapers from 0.310" to 0.314", before the crown. (More of a funnel now, I suppose.)
Based on that information from the Krag, I have decided to continue shooting jacketed bullets, but I do keep a close eye on performance. Right now, .308" bullets seem to obturate enough to make up for the .309" throat, and the 'funnel' crown doesn't seem to be affecting accuracy negatively.
For most rifles, all that matters is the throat/leade.
For handguns, however, the lower average velocities allow more bullet choices and adaptation, should an odd bore be found (like my .380).
I've never met a firearm that couldn't be slugged.
(I'm not including muzzle loading weapons here. I don't consider them firearms. Some can be slugged. Some cannot.)
"Such is the strange way that man works -- first he virtually destroys a species and then does everything in his power to restore it."