You're right - Boxer's homepage did actually acknowledge DOSBox until a couple of days ago, when I rewrote most of the copy and in the process took it out. I now realise this was caddish of me, and have added the acknowledgement back in.
It wasn't my intention to take credit for DOSBox's work; it’s an excellent emulator and responsible for 90% of what Boxer does. However, it's also extremely painful to use on the Mac - and I wanted to avoid the suggestion that Boxer's user experience was like that of DOSBox’s.
As airolson noted, DOSBox is also prominently credited within Boxer itself.
Boxer started its life as a sort of frontend but is now a fork of DOSBox. All of the changes are meant to fix (replace) the awful workflow that DOSBox currently has with mounting drives and tweaking config files and etc.
DOSBox is mentioned in the About window, the credits, and Boxer itself when you launch a gamebox. It also used to be on the website, though checking now I'm not sure where it's gone.
Boxer is properly GPL and you can find the source code here:
Boxer should at least acknowledge DoxBox on their homepage IMO, feels to me a bit like taking DosBox's glory for themselves?