A quick search brings up this RFC to remove green threads [1] which is dated "2014-09-16". So, it's entirely likely that Rust (sure, pre-1.0) had green threads around the time that the Java conversion happened.
Although the only important thing is that WebAssembly is 2015 [2] so there's pretty much no reason to convert to Rust to target WebAssembly when neither of them were actually released.
Yeah, to be clear, I'm not taking any position on what should have been ported. Just that you had indicated you weren't sure when green threads happened, and so I'd figure I'd chime in and let you know the details.
Although the only important thing is that WebAssembly is 2015 [2] so there's pretty much no reason to convert to Rust to target WebAssembly when neither of them were actually released.
[1]: https://github.com/rust-lang/rfcs/blob/master/text/0230-remo...
[2]: https://en.wikipedia.org/wiki/WebAssembly#History