more accurately, those canon milieu references say, "most are too lightly armored to stand in the line of battle." (Supplement 9 - Fighting Ships, page 9. There, I said it!) Same reference clearly says they're engaging in lighter combat: commerce raiding and orbital fire support are mentioned. So it isn't what he feels, it's what the game supplements say.
I see no great victory in blaming the messenger for what the game itself is saying. The point he's making is the same point you're making - those canon references about what the cruiser can and can't do are in conflict with High Guard rules. Depending on your point of view, that means either High Guard is broken for creating circumstances that contradict the role-playing milieu's realities, or the milieu's broken for saying things like cruisers "are too lightly armored to stand in the line of battle" when High Guard makes them more cost effective than battleships in that role.
I agree your reference ... but I'm not sure I agree your conclusion. "Fighting Ships" page 9 is headed "The Imperial Fleet", and is an overview of just that ... the IMPERIAL fleet. And when it describes what you should expect to find in cruisers, it is obviously referring to the cruisers of the IMPERIAL fleet, which the supplement then goes on to describe.
And, if we're confining ourselves to the cruisers in Fighting Ships then the observation that "most are too lightly armoured to stand in the line of battle" is plainly right. I mean, just look at them.
Gionetti class - unarmoured hull and no meson screen? Definitely a good idea to keep THAT away from the line of battle. But then again, it's Jump 5! Knock it down to jump 4 and give it Armour-10 and it's beginning to look more like it ... although I'd still want to give it a meson screen. Throw away most of those lasers and it could have a factor-7.
Arakoine class - totally unarmoured hull. But then, it is "specifically designed for ground support and surface bombardment", and was never INTENDED to stand in the line of battle.
Ghalalk class - the "basic fleet work-horse", but only armoured to factor - 5. I certainly wouldn't let THAT anywhere near my line of battle ... leastwise, not while there was any serious fighting to be done!
Azhanti High Lightning - Armour 5 and agility 0? Watch them fold up at the slightest provocation! But then again, like the Gionetti, they've given it jump-5 and you gotta pay a big prtice for that.
Atlantic class - at last we have a cruiser that CAN stand in the line of battle. But at Armour - 10 and Meson Screen 6 it is still questionable whether it truly "fulfils the basic design requirement of meeting the enemy and winning in battle" if the enemy were of the same tech level. Nonetheless, it's a shame it has been declared to be "fast approaching obsolescence", since it is so clearly the best cruiser there.
So, yeah, if those are the cruiser classes under consideration, then there's nothing wrong with the proposition that "most are too lightly armoured to stand in line of battle". But that doesn't make it a universal truism - and I'd happily put my 18KT J3 CLs in the line of battle any day, with their Armour - 13, good screens and J class meson guns. It is entirely possible to create cruisers suitable for standing in the line of battle, and Supplement 9 doesnt' say that it isn't, or you can't, or you shouldn't.