I came across an interesting speculation that basically wondered if it would be more cost effective to use cheaper jets as fighters in greater numbers. I thought it may be plausible, but I wonder if there is someone who knows for sure.
These days, fighters don't fight so much against each other as they fight against surface anti-air defences.
And those Citations won't stand a chance against even simple AA gun battery. They don't have manoeuvrability to zigzag to make them harder targets, don't have manoeuvrability to fly at tree-top heights to minimize time in covered space and have huge radar cross-section, so the targeting radar will lock them from miles away.
Avoiding these threats is what makes the fighters expensive.
The short answer is no and frankly the article says it all in the first paragraph...
Now, I don't claim to have any expertise on aircraft design nor to have done any serious research on this subject,
Not to mention that a new citation X costs 22.5 million (according to wikipedia) and the Mustang comes in at a mere 3.28 Million not the 2.5 million he seems to think. But still a a bit cheaper than some fighters.
But lets discuss it for a minute. First off military jets are far more complex aircraft and are capable of maneuvers that civilian aircraft simply are not. The cost of the jets is related in part to the complexity of actually building one and the fact that they often use simply more expensive materials like Titanium in simply more places than a civilian aircraft.
The author goes on to make some egregious claims about strapping weapons to a Citation. While this may be possible it would most likely cost more than anticipated not to mention large airframe modifications. If you simply are asking "can you put some big guns on a citation" the answer is in a since yes. Could you successfully dog fight with a citation, I doubt it.
Lets look at his comparison with the Bede BD-5J, your limiting factor here is simply gross weight. In a plane that size you may be able to carry a 6-shooter (and if you have been dieting all week) 6 bullets for it. Not making it a very effective fighter. This will come into play with the Citation as well but I dont know the loads they can haul so Im sure how much you will limit it.
The argument here of masses with lesser technology holds little merit since in a dog fight situation the fighter is going to be simply faster and more maneuverable and even if you had 10000 Citations with guns the fighter can simply out run/maneuver them.