I haven't read the Mallett comments because if he isn't talking about spin bowling I don't want to know what he has to say, but having heard Chappell talk about the way modern batsmen play the short ball in the past, I don't think he's tried to big-up himself (he doesn't have to anyway tbh, his record is pretty fantastic, especially once he became captain). I think he's just making a point about the changes in techniques since helmets came along. I think it's a valid point. I don't even know if he' saying the don't play it optimally, rather that from a safety POV it's not great. From a run scoring POV I would reckon the way modern blokes take on the short ball would be more productive than ever.
Likewise, if you look at clips of the pre-helmet days, batsmen stayed on the back foot A LOT more than they do now. You see blokes like Chappell, Sobers, Boycott and Walters to name just a few from their era, and they're almost on the crease to balls which blokes are way forward to now. That may just as likely be a product of playing the ball later and not committing to the front or back dog too early in those days as opposed to now when blokes seem to just pick the line and hit through it earlier, or it may also be part of the evolution of batting in the era of the helmet. Either way, it's a noticeable change imo.