I could never find any spell data confirmation of this. It's possible he originally cast
Lava Breath, which would put it at 500 (the spell id for Molten Breath translates to a Lava Breath on Live and they have the same text, ect.), but Molten Breath was in from the beginning of Velious and not used for anything else afaik, so there's nothing other than anecdotes to suggest the lower value. It was also described as easy to partially resist so the bad information could be attributed to that even though the source was very credible.
I don't have a ton of experience reading rawish spell data, but I think these are the relevant fields -
The first column is "Recovery Time", the last column is "Target", same dates as above. The recovery time obviously suggests 12s, but I'm unclear on how much the target field and the change on 8/22/2001 mean. It matches up with other target-centered AoEs like
Chaos Breath, but not PBAoEs with strict cooldowns like
Silver Breath and Lava Breath, which instead list "Point Blank AoE".
Hopefully someone more experienced with reading these can weigh in.