To be fair, Chiron's Noble Phantasm
is a meteor strike, and even
it wouldn't be enough . . . But yeah - the idea that, by homing in on
Mash's "home timeline" (since she actually
became a Servant), they could send an SOS to Chaldea, struck me as the "absolute overkill" option.
The fact that they
have actually defeated beings on Ymir's level would make it make sense
in-story, though; it's part of
their "legend" to overcome threats of that nature, and so on a
conceptual level (whether by TYPE-MOON rules or
Scion's), they
could win. As such, summoning them as a last resort
would be reasonable, narratively - just
equally ridiculously, narratively.