They're talking about MSET. So NTR does not have 8.1 access via MSET.NTR also has it but it's closed source.
BTW, NTR is not using ROP but CN exploit.
Also, NTR may be closed source but it's already been RE'd and is available to those in the know.
In addition to what @lemanuel said, you'd need to update in rxtools' "dev mode" which isn't yet working on new3DS.Once you have mset downgraded with Gateway, couldn't you just switch over to Rxtools with its dev mode and update that way? You should be able to run Rxtools from Mset (just install its ROP and proper files?). Or does it not yet support 8.1?