> What "shared resources" am I contending with previous developers for?

FTA:

> CI takes 30 minutes to run when it used to take 3? Fire everybody, and it'll still take 30 minutes to run.

You are contending with them (specifically, the code (tests, etc) they wrote) for CI runtime. The code doesn't go away just because the developer who wrote it is gone.

> all the remedies he suggests

Are specific to the low-hanging fruit of scenarios that do not include firing every other developer - I wrote "In a Hale-like model" for a reason.