The only way to know is have them post why somewhere. There are several rifle companies that thread 1/2-28 on 30 cals and call them suppressor ready. And many other suppressor manufacturers that seem to be silent on what size is required for their suppressors. Again, I don't have a 1/2-28 threaded 30 cal.
Should you believe the rifle manufacturer or one of two of the many suppressor manufacturers? Or, is the point to be as safe as possible, but in that case, shouldn't all calibers have the same requirement? For instance, I'm pretty sure a 7 mag or 7 PRC has more power and causes more deflection than a 7.62x39. Also, there is more leverage on a longer barrel/suppressor.
In any case, it would be nice how those two companies came up with that measure. Maybe they will post somewhere.