Yea, that's interesting, but I still bet this is likely a case of no one seriously looking into this term and knowing the full lexical space that well.
Boilerplate's early use was referring to boiler material, but it likely got semantically mixed together with the term for "standard form plate affixed to a boiler", as this term is actually more from the legal industry where it's literally a document with blanks you fill out, which more closely fits this definition.
I think people have looked into it and produced the most likely explanation. The name plate thing is a fun kind of 'folk etymology' theory but isn't supported by any evidence at all beside the hunches of people who've been around boilers. The other variant has a wealth of evidence supporting it.
Boilerplate's early use was referring to boiler material, but it likely got semantically mixed together with the term for "standard form plate affixed to a boiler", as this term is actually more from the legal industry where it's literally a document with blanks you fill out, which more closely fits this definition.