While that post was talking more about international outsourcing, let's examine this for a moment on a grander outsourcing scale since you think it's far-fetched for half the gates to be non-DL.

There 62 gates at terminal A.

12 of them can handle an international arrival.

Air France
Virgin Atlantic
AeroMexico (MEX)
AeroMexico (MTY, DL/DTW is losing that route to them in the near future)

The remaining 50 gates at terminal A are often populated with CRJs and EMBs. There can easily be eight of them at once on each inner alleyway, for a total of ~16 plus the possible four aforementioned JV flights on the other side for a total of 20.

20 / 62 = 32% outsourced

But that is if every other gate of the remaining 42 is occupied by a DL mainline jet.

If a third total were open, or about 20, then 20/42 is very close to half of the jets parked at terminal A being outsourced/JV metal.

Oh, and there's the entire B/C terminal which is of course 100% outsourced flying.

Less than half of DTW departures the past few years have been mainline. Good to see it creeping back up on the small aircraft end of the spectrum, but the JV international stuff is growing at the other end of it.