Some bad news here maybe. All our characters have "counts as" armor. The rules say that you only get to buy magical armor or a magical shield if you already have or can buy a mundane equivalent. The question is if counts as armor is the same as armor. If you say that it is, that leads to some weird interactions. Suddenly, your Skink Priest (which from a rules perspective has exactly the same armor as an Oldblood!) gets to buy for example Armor of Silvered Steel, which i do not think is intended. I also find it kinda telling we have no magical armor at all in our book, almost every other army has it. Other armies that have counts as armor, like Woodelves Treemen, generally agree that they do not get to buy magical armor, we are just in the weird place that all our characters have counts as armor. What do other people think about this?
Lizardmen never had any magic armour in any book since 6th edition (probably only the hide of the cold one, which was an heavy armour and now give +1 instead), they simply brings back to Tow old magic items. I think people need to stop rack their brains to over complicate thing that are easier. priest CAN buy magic armour, but cannot cast if they do so (because only callous hide allows them). Other character can simply buy armour. Because, if we REALLY want to read RAW, no one can buy helm, or chaos lord cannot buy an armour of destiny (because he only have plate armour, not a heavy).
I mean you could indeed go even further and argue that if you have light armor you can only buy light armor equivalents, idk if i would go there, but you could. I am personally unsure what the correct ruling is, i think it is good to talk about before a game though.
Every single person I've ever met has agreed that "counts as heavy armor" means "counts as heavy armor for rules purposes." Honestly, what else could it possibly mean? "Counts as heavy armor for some rules, but not others, but we're not going to tell you which" is not a very plausible interpretation.
In terms of Old World - I agree with this. As an additional info, just for fun - RP/history-wise - LM can and do wear armour, usually golden. WH:Roleplay Lustria (it's an official WH game) : "Some Saurus strap on headdresses and golden armour as a mark of rank and ceremony." "Lizardmen do not like to wear steel plate or chainmail armour, finding the experience one to which they have a strong natural aversion. A Lizardman could conceivably be forced to wear such armour, but that armour would count twice its ENC (rather than half) when worn by a Lizardman."
Add to that the old blood carno model physically comes with different pieces of armour to equip him with and its tough to say "you can't buy armour" from a wysiwyg perspective
Although I believe buying armour is obviously both RAW and RAI I'll point out that we are unsupported in TOW and you're looking at AoS models.