I find that kekahi does translate to a/an within a sentence. Per Olelo Oiwi in the Glossary. Also noted in the usage of the word on page 121.
Ex: Makemake au I kekahi puke olelo Haole. Trans: I want an English book.
When I read the response that kekahi does not mean a/an in any case it caught my attention as I remember seeing kekahi noted a/an in multiple sources on line. Any input on this is appreciated. Aloha Kakou.
Pololei ʻoe e Brandon. Also in Pukui's dictionary, as well as in the much older Andrews dictionary.
I think the confusion in DL may have come from the fact that kekahi is commonly translated as a/an (within a sentence where "he" doesn't work), but people tend to forget that it can also mean "another," so DL seems to be trying to reinforce that usage.
For beginning language learners, however, the meaning a/an also needs to be taught and reinforced. Language learners tend to use "kekahi ʻē aʻe" for another, but that's actually redundant. You're observant, so have a lingot! :-)
P.S. You obviously have the original version of ʻŌlelo ʻŌiwi, but the revised edition should be coming out by the end of the year. Fingers crossed!