Yeah, Rabbi Yochanan is way off. That would make a sqaure of about 5.4019 tefachim on a side. (24/(pi*sqrt(2)) (also, sefaria incorrectly writes "circumscribed" when they mean "inscribed") OTOH, the sages (or the translation) are off, too, since they underestimated both pi and root 2. The actual circumference should be should be 17.7716 (rounding up, 'cause halacha), so they are off by a tefach.
(I think R' Yochanan misinterpreted the Caesareans. I think it likely they were referring to the AREA, not the circumference/perimeter. for a 1x1 square inscribes in a circle of area ~1.5708, or about a "square and a half" Some things are better left to mathematicians, Rabbi) (pi/2)
What did I JUST SAY about math!
No more math unless it is couched in the form of a limerick.
Yochanan said five and a bit,
But may have mistranslated it
And according to Pi
The circumferi
Should be more than a tefach more, dig?