First, let revisit why "I could buy a ticket yesterday." is incorrect. The issue here is the tense. The word "yesterday" fixates the tense of the sentence to some form of Past Tense, because "yesterday" can only refer to the day before (now). So if you keep the word "yesterday" in the sentence, you must change the tense of the verb. "I could have bought a ticket yesterday." is correct. Of course, if you drop the word "yesterday", again the sentence is grammatically fine, "I could buy a ticket". The other sentence may be a bit tricky. "this morning" does not really tell you whether "this morning" is before (now), or right now. Many of times, "this morning" is in the past, but not always. For example imagine that the person said that sentence during the morning, and the morning is not over. That means one must use Present Tense, which makes the sentence valid. If the person told the same sentence in the afternoon, then the sentence would be grammatically incorrect.

I could not find your quote anywhere else online. In cases like this, it's better to provide a link or even a name, instead of "a respectable author". Anyway, yes the sentence is correct. Perhaps you're still confused about the difference between "could" and "was able to"?