Sounds to me like Myth! the radar head sweeps at 10-12 RPM so your transponder will reply to that radar at 5 second intervals so what are the chances of a pilot tuning the transponder through an emegency code during the time that his transponder is being interogated by a radar (remember the interogation pulse is fractions of a second).
Unfortunately - or fortunately, depending on your perspective. - the technology used in radar systems (the ground bits anyway) has moved on a bit. A modern system tracks each target and will make some 'assumptions' if the code returned by a target's transponder changes. Equally, the picture displayed to a controller may well be formed from data collected by a number of sensor heads.
At the same time, both ground and airborne parts of the system have been modified to overcome problems caused by pilots and controllers not following simple instructions or thinking they know better than the people who built the equipment. Such modifications on some systems include things like the suppression of mode A data (either sending it or displaying it) when it changes until the data is again consistent and not displaying an emergency code until there is confidence that it has been selected intentionally, for example, it has been sent for two sweeps or received by two separate sensors.
The bottom line is that
some combinations of equipment benefit from switching the transponder to STBY before changing the code, although most now can cope with not doing so. As Shy says, RTFM if you can and do what it says.