vpn software is pretty rigidly defined, they all pretty much work the same. Only the SERVICE they provide is unique or better/worse.
Odd are good they are using a standard L2TP or IPSEC vpn. they will have "manual setup" instructions on their support site, so you can connect from ANY device using its built-in vpn support, without having to use their custom client software. (which won't be available on every platform - ie they may not have a smartphone app for your phone, so just 'do it the hard way' and use your phone's built-in vpn support)
this same manual setup process is used to configure a *hardware* vpn box that sits at your cable modem, and creates a private LAN on the other side of it. (it functions as your router, with vpn functionality by default for everything on your local network)
again they can't write a client for the box, they are just using one of the standards, and you just go into the box's admin setup and plug in the values and turn it on and it goes.
