This is ironic because Spirit also manufactures parts for Airbus, etc.
"As a result, this check job that should find minimal defects has in the past 365 calendar days recorded 392 nonconforming findings on 737 mid fuselage door installations (so both actual doors for the high density configs, and plugs like the one that blew out). That is a hideously high and very alarming number, and if our quality system on 737 was healthy, it would have stopped the line and driven the issue back to supplier after the first few instances."
My understanding as well is that while Spirit makes the fuselage of the 737MAX, final assembly is at Boeing in Renton Washington, and the plug door would have been removed for final assembly and then reinstalled in Renton, so Boeing itself would be responsible, assuming, as appears likely, that the plane left the factory without the door bolts installed.
Plenty of room for differences between those factories and the former-Boeing one in the USA.