Of course it may not be CMOS at all.
When "limited ESD protection" is written it usually means a pin can withstand voltages higher than its supply voltage. In other words there is nowhere for input clamp diodes to conduct to. It is quite common in 1.8/3.3V powered MCU when they have 5V tolerant input pins and the usual protection diodes are omitted. It is less common in other devices but probably for the same reason.
Not surprising to me. Many old bipolar linear had no designed ESD protection at all. I once worked on a RF SOI mixer (FET-quad) that had none, and a 10V (yes) ESD failure threshold.