Although the application itself is not open source, the developer has open-sourced a restore tool, to help ease the "what if they go out of business" concerns:
https://github.com/sreitshamer/arq_restore
Also, somebody else has created a Go utility that can read the backup format:
https://github.com/asimihsan/arqinator
Finally, if you really wanted to do it all yourself, the data format is documented here:
http://www.haystacksoftware.com/arq/s3_data_format.txt
Although the application itself is not open source, the developer has open-sourced a restore tool, to help ease the "what if they go out of business" concerns:
https://github.com/sreitshamer/arq_restore
Also, somebody else has created a Go utility that can read the backup format:
https://github.com/asimihsan/arqinator
Finally, if you really wanted to do it all yourself, the data format is documented here:
http://www.haystacksoftware.com/arq/s3_data_format.txt