I almost added an example like this to your first post, which I think would be more natural for me:
(b') I ate some delicious apple pie at this restaurant last week.
Tho it is not at all wrong grammatically, I would not choose or use (a). (I don't think so, anyway.)
Example (b) makes me think back to some time I spent in new zealand, where, for lunch/take-out, you could order various kinds of pies. I don't remember apple pies there--or other fruit pies--but they did have meat pies. These were one-serving creations, smaller and intended as one serving--a single pie for a person. They were a common lunch item for school kids, and looked something like this:
https://www.taste.com.au/images/recipes/wfr/2005/09/13935.jpg tho sometimes they were bigger and would be split/shared so that different people would get a portion of one.
There, I think "pie" was a word used instead of what an american would call a pastry. So there, each person could (and commonly would) eat a whole pie, while eating a whole pie (one pie) is something americans would find really really strange.
If "pie" is taken with a broad meaning, then "I ate a delicious pie", like (b) seems okay to me. That would be a single-serving kind of pie. But if "pie" is used in the american sense, saying "I ate a pie" would shock most people, since it sounds like you might have eaten 6-8 slices of (typical american) pie all at once.
https://www.tasteofhome.com/wp-cont...y-Pie_EXPS_BW19_29181_E08_22_2b-2-696x696.jpg