So these food objects could be plural or singular depending on context?

Yes, that's right.
It depends on the context.

I understand that when "sebuah" (one/a/an) is written it eliminates the ambiguity...

Yes, that's also correct.
When "sebuah" (or another counter/classifier) is used, then it should be obvious if it's singular/plural.

For example :
Kamu makan sebuah apel = You eat an apple.
Kamu makan dua apel = You eat two apples.
Kamu makan banyak apel = You eat many apples.

Just to add on. For fruits, the counter word usually used is "sebiji". Sebuah is usually used for big things for example house, country.

Sebuah rumah = a house Sebuah negara = a country

Here both singular and plural should be accepted.