Nice joke, isn't it! It plays with the pronunciation of the terms horse and whores. In General British (GB) the former is (phonetically) hɔˑs and the latter hɔːz̥. (Some General American (GA) might have hoʊɹz̥ or hʊ(ə)ɹz̥ for whores.) As you know, in horse the vowel is clipped (= shorter, pronounced more quickly) because it is followed by the fortis consonant s (hence the symbol ɔˑ in the transcription). In whores, on the other hand, the vowel tends to be longer as it is followed within the same syllable by the lenis z, which is devoiced because it is on the end of the word and there is no other voiced sound coming after it: z̥. Have a look at this amusing YouTube video clip.
Take the famous example sheep and ship: in both words the vowel sound is short(er) because both words end in the fortis consonant p. Phonetically speaking, the main difference between the two terms is not so much in the length of the vowels (they're both clipped) as in their quality: the former is (ɪ)i, the latter ɪ. The use of the lengthmark to represent the vowel in sheep (= iː) is therefore utterly useless in this case. Jack Windsor Lewis has already discussed the confusion the use of lengthmarks may cause for EFL students in this article of the 25th of April 2014. In his Concise Pronouncing Dictionary of British and American English (OUP, 1972), now sadly out-of-print, he transcribes sheep as ʃip (p. 184) and ship as ʃɪp (p. 184).
Readers of my book can practise pre-fortis clipping and devoicing of the English lenis consonants on p. 13 of my L'inglese medico-scientifico: pronuncia e comprensione all'ascolto (EdiSES, 2014):