Approximately 0 English speakers use the long scale. Exactly 0 do in the US.

You're completely forgetting about non-native English speakers who use the long scale in their native language. They make mistakes or might even not be aware of the difference.

I wouldn't advocate using these abbreviations in a technical document that has safety implications, but in an S-1 it's fine. If you make a mistaken trade because you can't keep your scales straight I don't really have a lot of sympathy. If they don't know the difference in the first place they're not reading this document.

Metric is not universally known in the US, either, so the SI unit prefixes are also not universally known.