The thing is, unless you are severely undergeared, there's just no moment where the throughput of holy will make up for it's weaknesses (prone to overhealing, mana intensive etc), while disc is good at both lower gear levels (not reliant on spirit, heals "unhealable" damage with absorbs) AND high gear levels (Scales a lot better than holy because of the no spirit thing).
So yes, he's completly right. Assuming the disc and the holy priest are both equally as good as each other, there's absolutely no situation this tier where you would pick the holy priest over the disc. Not even malkorok, considering that fight is pure overhealing, and thus shows nothing except raw potential output (that'll never be realised) which is irrelevant*.
All that aside, the highest disc on malkorok sustains 610K vs 750K in 25 man, and 350K vs 393K in 10 man. This is not even *close* to being enough to drop a healer in either raid size - simply put, if the fact that a holy priest has 10% higher throughput in 10, or 20% higher throughput in 25 means you are able to drop a healer, then you WERE ALREADY able to drop a healer.
Oh, and also, why do you mention thok? There's one holy priest in the top 100 on Thok 10 heroic, and two in 25 man. The best 25 man is about 20% behind the best disc, and the best 10 man is 30% behind the best disc. That fight is a *paradise* for disc.
*(Take the #1 holy priest, Amity, from Vodka - 750K hps. Their raid did 490M healing combined, but the raid only took 265M damage. Even assuming every single raider had 1M HP, that's still 200M healing more than there was available to heal up).
Everything said here aside, people keep saying "if your raiders perform well", and "if you don't take lethal damage". I mean, that's just not a likely scenario at all. Mistakes will happen, and disc can save people from those mistakes. Our raids survivability took a noticeable hit without a disc priest because sometimes you're just gonna get fucked over by laser+drill+richochet on juggernaut (as an example), and there's nothing any of the other healers can hope to do about it. It isn't even because of bad play - it's because the game is designed to try to burst people down quicker than healers can react.