The domNode manual page says the signature for the getAttributeNS method is:
<ul><pre>getAttributeNS uri localName
The same is reported when giving an invalid method to a dom node command.

However, when trying to pass a default value, the command throws an error:
<ul><pre>% $node getAttributeNS $uri $attr $default
wrong # args: should be "domNode0x55ba42ccfb60 getAttributeNS uri

I found this on 0.8.3, but looking at the latest version of the source code, I am quite confident the situation is still the same.