Revising the various usages of “als”, the website thegermanprofessor.com lists in section "(3) than (comparative conjunction)" the usage of “niemand als” as “nobody but”, unfortunately without an example.
Checking on the Internet, I have only found versions which use außer (e.g. Duden).
Außer dir habe ich keinen Freund.
Could this usage of „niemand als“ be a regional phenomenon?