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.
Is that including people who immigrated or only those who grew up there? Your link [0] mentions "residents" throughout, which implies that it's counting Chinese people who moved to Singapore as adults. I've known a decent number of Singaporeans, including many ethnically Chinese Singaporeans I met while working in Beijing, and I've never met a single person who grew up there and can't read English.
If adult immigrants are included, then I strongly agree with the later part of your comment! There's no reason the US shouldn't similarly be included in the list as it has tens of millions of Spanish-speaking residents who immigrated as adults. Canada, Australia and New Zealand should probably be included too, if Singapore is.
I also assumed it was including mainland Chinese immigrants, especially since the rate of Chinese only is so much higher than Malay only and Tamil only. Lots of wealthy Chinese move to Singapore every year. (A bit different from immigration in the US.)
[0] https://www.singstat.gov.sg/-/media/files/publications/cop20... p. 42
[1] https://www.ef.edu/epi/
[2] https://www.census.gov/content/dam/Census/library/publicatio... p. 8
[3] https://news.yahoo.co.jp/expert/articles/d4a8d621480672a1c99...