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Spoiler for mechanicum.

Hopefully they'll expand on it when...

It's also possible he kept it imprisoned in a undisclosed, sealed place until there were significant tech developments for it to be transported there safely. Also, given that the Emperor was aware of the Webway gates on Terra, it's possible he used it to get it to Mars. Hell, I wouldn't put it past him to use his psychic powers to transport himself and the dragon to Mars either.

You're not supposed to read the account of the battle literally.
It's internally inconsistent, doesn't make logical sense, is described by the character telling it as being unreliable at best, and similar stories are described by the main character as metaphorical earlier in the narrative.

EDIT:
Page references:
https://www.warseer.com/forums/showthread.php?223315-Iron-Hands-and-their-loyalty&p=4009747&viewfull=1#post4009747