Probably, yes. Did you report it using a "my answer should be accepted"? That is the quickest way to get the team to look at it.
Technically the above is a valid translation too. I can think of expressions where I would definitely, as a native English speaker, say "I have had..." For instance, "I have had enough of this nonsense". In Welsh that is "dw i wedi cael llond bol o'r lol hyn".
I think they should just accept either here.
Now you mention simple past, I have to rethink!
Wedi is always used to form the perfect tense (present perfect, future perfect, conditional perfect and pluperfect). "Dw i wedi" is the (present) perfect, and cael is the verb "to have" that is being placed into the perfect tense. I thought "yesterday I had coffee" was fine because it is clear that this is a finished action, but now I think about it, it is not strictly the present perfect. "I have had coffee" is correct here. That differs subtly from simple past, for which you don't use wedi.
Simple past is denoted by conjugating the verb, or using "bod" in the simple past (a tense that English does not have, but there is a lesson on it here).
Ces i goffi is "I had coffee" and I retract my agreement that "I had coffee" is a better translation here. It does literally mean "I have had coffee". Sorry!
You're right. "Yesterday, I have had coffee" isn't good phrasing in any variety of English I know of. Since yesterday is in the past, the past perfect is an option, but only if you want to describe some sequence of events or give more time constraints on something that you finished doing within the previous day, as in "Yesterday I had had coffee before eating breakfast" or "Yesterday I had had coffee by noon."
On the other hand, I definitely agree it's important to know that Welsh uses present (? actually, I think wedi is an infinitive, but I don't know) perfect in this context, so it makes some sense that they also give the literal translation (if that's really what's going on). I think what would help is if there were a heads-up in the lesson's notes.
For what it's worth, for contrast, I wonder how, "Yesterday I had had coffee by noon" and "Yesterday I had had coffee before eating breakfast" would be said in Welsh.
Edit: This got addressed here. Apparently wedi is a preposition meaning after, so I guess these sentences just need to be fixed in English. And I guess the past perfect in Welsh might be rendered like, "I was after..." "Roeddwn i wedi..." maybe.