Perhaps that's because the K-Ar dating method is only accurate for samples 4.3 billion years old to around 100,000 years before present.
This is no secret, except, apparently, to the author of this article.
That's not a logical argument. You are saying that you have to know the age of the rock and be certain that it's over 100,000 years old, before you can rely on the K-Ar method. If YEC's are right, and no rock is over 100,000 years old, then every K-Ar date is bogus.
The problem with the K-Ar method is that it assumed (falsely) that all of the Argon boiled out of the hot lava. That was a bad assumption and has been proven false. Excess Argon has been reported in recent lava flows from at least 16 recent and different lava flows.
Accordingly Scientists switched to the Argon/Argon method which supposedly is not dependent on Argon boiling out. However, doubts remain.