I think it makes sense to include it because not all Singaporeans are literate in English. For example 17% of Chinese Singaporeans were only literate in Chinese according to the 2020 census report.[0] Additionally, Singapore is not even highest on this list; Netherlands and Norway are slightly above it.[1] However, by the same token, the US should probably also be included due to its high level of non English-speaking immigrants.[2] It's worthwhile to note, however, that the EPI does not test a random sample of the population so the usefulness of these results is not certain.[3] This would be even more of an issue in the US were almost all EPI testers would be immigrants, so the results would look much lower than they actually are.
[0] https://www.singstat.gov.sg/-/media/files/publications/cop20... p. 42
[2] https://www.census.gov/content/dam/Census/library/publicatio... p. 8
[3] https://news.yahoo.co.jp/expert/articles/d4a8d621480672a1c99...