He means relative to Nitrogen you have about 4X more if you dose KNO3 before you become K+ limiting.
Typical N to K+ ratios are about 1-1.5 to 1.
Since NO3 also has a lot of Oxygens, that reduces the ratio down to about 4:1 N:K.
Also, KH2PO4 adds a little, GH booster/SeaChem Eq is about 30-50% K2SO4.
So you really do not need to add it from other sources.
However, doing so will not help either.
There were a bunch of folks in the early 2000's that claimed excess K+ caused problems, but I argued it was something else, not K+ and demonstrated it specifically. Still, they where unconvinced but never supported their reasons. But if it was true, we'd seen and heard more about it, but that's not been the case.
Folks go from one nutrient to the next blaming it for this or that, this comes up and makes a few rounds every few months or years.
If the nutrient is non limiting, then you can move on and look for another reason.
But you have to know what non limiting is for the system and perhaps a good idea to know what the upper bounds are as well.
But I do not know what the upper bounds for K+ and PO4 really are.
Well over 100ppm for both.
Non limiting about 15-20ppm for K+, likely less, and PO4 as long as some is present at 0.2ppm or higher as SRP.
So it really is a huge wide range, which given the wide range of environment conditions plants deal with, something we'd predict.
Still, some think there's some narrow precise range the nutrients must be maintained at, which flies in the face of logic, plant science and observations.
There are much bigger fish to fry here.
Light(Virtually never measured, unlike NO3 etc) and CO2(very poorly measured), general care/cleaning/filter cleaning/good gardening/fish loads, feeding, consideration of the light to CO2 to nutrient load.
These things can do far more damage or help to a planted tank than nutrients, but few bother to really go after and measure them.
Getting decent nutrients and keeping within a pre set range(you chose whatever that might be) is really pretty easy.
Very critical measurement of CO2 without interference, or light measurement are much tougher. And these are what drives a tank, nutrients are downstream of these, but still critical.
If you cannot compare light to a different tank, or CO2, that obviously will cause changes to any conclusions about the nutrients. Still, even with this glaring hole, many still try
Some do account for light to some degree, or have decent CO2. Some know what good CO2 looks like because the nutrients and and the light are fixed variables. The they adjust the CO2 to an optimal level using the fish and plants and they also have good current, prune routinely, good flow, some surface movement etc.
CO2 has a number of issues. It's not so simple as some suggest.