Joe,
You are right to be sceptical.
When I see the words ‘cast steel’ or ‘warranted cast steel’ on older edge tools, I take it to indicate that the item was forged from crucible steel.
I would assume that the same applied to Klingenthal blades. I shouldn’t assume that, but it seems so improbable that they would cast blades as blades, and so important a revelation if it was true, that the author should provide supporting evidence.
Apart from anything else, why on earth would anyone cast a fine blade? It would need a lot of finishing, and in any case would likely contain porosity, inclusions and other defects.
As for photographs of molds, under what circumstances would a mold have survived? A pattern, yes, but a mold, I hardly think so!
I did a quick search on Klingenthal and acier, and found an 1832 source concerning the manufacture of blades at Klingenthal in acier fondu (molten steel, i.e. cast steel), but it discusses
forging temperatures.
Interestingly, in addition to acier fondu, and acier cementation, the source refers to something called
acier naturel. It seems that this was made from ores found in certain parts of France, Austria and Sweden.
Source: 'Encyclopédie méthodique: Supplément au dictionnaire de l'artillerie' by Gaspard Herman Cotty:-
Encyclopedie methodique: Supplement au dictionnaire de l'artillerie - Gaspard Herman Cotty - Google Books
Joe’s original question, of when were steel castings first viable, is an interesting one, which I might come back to.