Realistic is very context driven.
Firstly, the article ddidn’t (from my admittedly quick skim) mention how they determined muscle gain. If they literally just put these participants on a scale at the start and end of their training then no I don’t think these results are unrealistic at all.
From the photos they all look to have gained fat.
Secondly, circumference measurements are usually by tape, not an error free measurement. Also, were they both taken post training? Was one pre one post? An untrained person would expect to gain some circumference just from inflammation and residual tone from starting training.
Thirdly, these guys all look like they’d never touched a weight when they started, whats realistic for a beginner is different to what is realistic for someone with training under the belt.
Finally, stop using other peoples progress to assess what is and isn’t realistic and what you should do. Try it, look at what results you get.