He means at the very end of the word. Oi in ψήφοις is long, because it's not at the end of the word. Now there are some exceptions, but final -ai and -oi are usually short. (The exceptions that come to my mind now are optative verbs that end in these diphthongs and rare locative forms such as οίκοι 'at home' long -oi and thus acute accent, compare οἶκοι 'homes' with short -oi and circumflex accent. The accent actually tells you whether the last syllable is long or short when the accent is on a long second last syllable, as it is in this case - in this case, the accent must be an acute if the final syllable is long and a circumflex if the final syllable is short.)
What is important is that this short final -ai/oi rule DOES NOT APPLY to verse scansion, it only applies for accentuation. Syllable quantity in verse scansion and vowel length for the purpose of accentuation (which we are talking about now) are TOTALLY INDEPENDANT. Syllable length in verse scansion has it's own rules that are slightly different.