I think that's essentially right - if it's already "thinking about" apples, then ' an' will be significantly upregulated at the expense of ' a', then if it does choose to output an ' an' token, ' apple' becomes a highly likely followup.

You could probably test this by seeing if prompts containing a lot of nouns that start with a vowel sound results in output that contains a higher proportion of otherwise unrelated first-vowel nouns. (ie your prompt includes lots of apples, apricots, avocados, asparagus, aubergines, elderberries, eggplants, endives, oranges, olives, okras, onions and you count the proportion of non-food nouns in the result that start with a vowel and non-vowel sound).