McHardy's book is certainly readable, I entirely agree there with your previous correspondent, and he is also right to stress the certainly underestimated tribal nature of early Scottish society - but repeating this ad nauseam makes it often very tedious. Of course, the Picts originated from (or were) the indigenous Scottish population; but what does indigenous mean in this context? The Broch builders, certainly, but the builders of megalithic monuments? Surely, McHardy does not assume that they spoke Celtic. So did Celtic drift into the British Isles on the wind?
It also seem spurious to assert a cultural equivalent between illiterate tribal societies and antique culture, however cruel Roman warfare was. What about Plato, Socrates, Homer, Cicero, Virgil, etc. etc., what about Roman engineering like the aqueducts? It almost sounds like "what have the Romans ever done for us?" ...
I agree with McHardy's assertion that there was no Scottish takeover leading to the end of the Picts (this is almost orthodoxy by now) and on the crucial role of the Vikings. It also appears likely that Fortriu was in the Moray area, although it does not entirely tally with the importance of Forteviot in the later period (and Scone, which he does not mention). I also find it unlikely that Pictish as a language disappeared very quickly, but the geographic evidence (place-names etc.) points to Gaelic becoming more or less the exclusive language - for a time. But it is hard to believe that the Picts (or some of them) spoke an early version of Scots. After all, the Angles spoke nothing like English but Old English, which was almost identical to Old High German - see Beowulf, Battle of Malden etc. for reference.
The most curious aspect of this book lies in the bibliography. McHardy ignores large swathes of the best modern Pictish scholarship. No Fraser, no Woolf apart from the essay on Fortriu, no Aitchison with his excellent studies on Pictish and Scottish warfare and on Forteviot, no Cummins or Clarkson, no Carver on Portmahomack. No wonder that this "New History of the Picts" offers hardly any historical narrative. We are further than that with the Picts.