I was also concerned about this when I was looking into Arq, so I wrote a cross-platform restoration tool that'll also work on Windows and Linux (not just Mac): https://github.com/asimihsan/arqinator

This is purely based on the author's excellent description of the format in his arq_restore tool: https://www.arqbackup.com/s3_data_format.txt

No it doesn't. They've had an open-source CLI tool since at least 2013.

https://github.com/sreitshamer/arq_restore

I just started using Arq, but it looks like the format is documented here https://www.arqbackup.com/s3_data_format.txt and there's some open source tools that can work with the format. https://github.com/asimihsan/arqinator and https://godoc.org/code.google.com/p/rsc/arq/arqfs