I've never heard this until you posted it, so I decided to check it out (I own a CZ 75B in 9mm and a SIG 226 in 9mm).
I could see where the idea might take hold at first. Both mags appear to be the same size, the same angle, and at first look the mag catch appears to be in the same place (though the CZ has only one and the SIG has one on each side).
On closer inspection the mags are definately different. The SIG 15 round mag is much heavier than my CZ 15 round mag. The CZ mag tapers to allow only one round at a time much sooner on the mag (the SIG mag is double stack until the last moment, maybe half a centimeter, the CZ mag is nearly single stack for the top inch). The CZ's mag catch is smaller and actually on close inspection about 1mm lower than the SIG's.
The SIG mag was not able to insert all the way into the CZ (it stopped short by about an inch). The CZ mag made it further into the SIG but still stopped short about 1/4".
Sorry.