Article 250 covers the "exothermic welding process" and 110.14(B) covers splices.
Here's that rule:
(B) Splices. Conductors shall be spliced or joined with splicing devices identified for the use "or by brazing, welding, or soldering with a fusible metal or alloy.
Soldered splices shall first be spliced or joined so as to be mechanically and electrically secure without solder and then be soldered.
I sure hope this isn't interpreted to mean that welded splices are OK for insulated conductors!
Interesting question. Maybe the code needs to be revised.
I didn't read the NECH commentary, maybe it has some more explanation on what is meant by "welded"??