TL: DR at end
I think the term efficient has been misinterpreted.
I think what the previous poster was trying to say is that holy isn't 'mana efficient' when tank healing. not that your spell choices don't make for effective tank healing. Sure, trying to tank heal using CoH and PoH would be dumb, but when using the 'right' spells to tank heal it eats through a fair chunk more mana than disc. Most bosses will be hitting too hard for 'heal' to be of any significant use. I doubt any priest has enough regen to support spamming flash heal so that makes gheal the main go-to tank healing spell for both disc and holy. As disc you inner focus, combined with ToT that massively increases the mana efficiency of gheal (works out that every 6th gheal is free if spamming?). You also have grace, I consider the holy equivilant of test of faith. 25% constant on a single target must outweigh 12% when they're under50% (raid healing ofc you can't have grace up on so many people, so test of faith really shines once groups of people drop under 50%, healing when you need it). You also have penance vs HW:serenity. Neither are bad although I often feel I'm risking the tanks life actually spending the GCD on HW, even if the crit afterwords makes up for it, on the plus side it's instant so nice for movement (although so is PW:S, although honestly for single target throughput renew is effectively the PW:S for holy), penance gives a pretty nice boost to HPS and also has excellent HPM. I'd argue that guardian spirit is probably the stronger tank CD compared to Pain suppression, but negligible aside from gimmick fight mechanics. Inspiration uptime should be about equal for both. Crit chance should be higher as disc in the same gear (same crit from talents, increased int from disc). Echo of light and DA can also be compared, both have strengths and weaknesses, if your tank isn't in danger of being 2-3 shot then echo is just as good on a single target imo, more reliable at least. potentially wasted as overheal.
Overall I'd say the two specs were fairly comparable with effective throughput. but disc just has a lot more talents boosting it's mana efficieny using the same or equivilant spells. (strictly single target healing)
PS, i didn't mention power infusion or serendipity.
TL: DR, both specs have comparable throughput abilities, disc has more mana efficiency talents.