"Grandfather" is spoken of /thought of in respectful, honorific terms. In Hindi grammar, this means using plural forms (even though there is only one grandfather).
As a way to remember this phenomenon:
Think about how the informal you, tū, is singular (hai). However, the formal/respectful you (āp) is plural (haiṅ).
Both make sense! But the sentence that you give is more precisely दादा सेब को खाते हैं The को makes it clear that सेब is a specific (THE) apple.
This is an aspect of grammar that the course does not cover.
They seem to have dropped the को (which I applaud since I think it is mostly for animate objects). Which means it could be one or more apples.
How do we know from this sentence that he eats more than one apple? Why not "Grandfather eats an apple"?
That is perfect correct as well. You really don't know how many apples there are. Above, RanzoG says that placing a को after the सेब clarifies that it is the apple. However THE apple is different from AN apple in terms of specificity. According to my sources (books and Hindi speakers) that को is not necessary for 'an apple'. It is primarily used after animate objects. However, as with so much about language, it might be a regional thing. Also, the rules about using को are fairly complex, so I could have gotten it wrong. Certainly, you could add the को if you wanted to emphasize that there was only ONE apple.