Verification, please?

Try not to overthink. I think the way it was wired this way is that Ma discovered "it would work" and they didn't have to modify the ignition switch or add relays, etc to supply the ECU

The other issue is, what with harness/ connector deterioration over the years, I would be using a relay triggered by the original output from the bulkhead to feed all underhood loads, anyhow. So with an MSD, just cut all that loose, wire the IGN1/ IGN2 together, and trigger a relay, with a big "ultimate protection" auto reset breaker.

Then feed off the regulator, alternator field, and your MSD trigger off the relay

Most of the reason for this is voltage drop in the harness/ connectors/ switch, which causes over voltage at the battery from the alternator.