In its current form Ninjhax has no CSND support on the New 3DS, so there's nothing you can do except:
A) wait for KARL/OSKA
B) wait for smealum to update Ninjhax again
C) improve Ninjhax on your own so it gains higher permissions for CSND without the browser (since SKATER has no CSND access for whatever reason)
D) get a Gateway and enjoy CSND on the New 3DS via homebrew CIA files
He was probably running it on a private CFW with CSND access rather than adding such to Portal itself. I guess if we could use memchunkhax to escalate permissions, gain access to CSND (this would only need ARM11 kernel, right)?, and then somehow cleanly return to the 3DS OS/homebrew launcher, it might provide a nice interim solution until one of the CFWs comes out. I'm not really a reverse engineer though, more of a regular homebrew dev, so someone like WulfyStylez or ShinyQuagsire would be more qualified to talk about that.