You can take things either way - there's moments during the 1st and 2nd Doctor periods where it's a strain to do the retcon and assume he does have two hearts and it didn't get noticed (the hi-tech medical checks in Wheel in Space and Seeds of Death, for instance). But there are occasional moments after Spearhead from Space when people check the Doctor's health and don't remark on anything odd.
One simple answer is that if a Time Lord can temporarily shut down both hearts to feign death (Destiny of the Daleks), they can also shut down one of them to avoid awkward questions (and that this sometimes happens by accident while in a coma). Aside from anything else, this could make Harry seem less of an idiot in Robot: he's not surprised that the Doctor's got two hearts, but is puzzled that it's now starting working, after being out of action while the Doctor was unconscious.
But anyway, the on-topic point is that Coifer hasn't got it wrong - he's ignored (or never heard of) a fan theory, and gone for the lore that modern kids know.