It just dawned on me that there are some CBP preclearance checkpoint overseas.

Are these checkpoint equivalent to local ones? In other words would I need to reclear CBP or TSA when I land stateside?

With pre clearance checkpoint you arrive at what is basically a domestic terminal. So no need to re-clear CBP or TSA.

CBP preclearance locations do have one practical difference for US citizens: onward travel to the US is de-facto at the discretion of CBP.

A US citizen at other points of entry may be detained for secondary screening, and may have his or her belongings searched or seized, but ultimately they cannot be refused entry to the US. At a preclearance checkpoint, CBP may not be able to technically deny entry to US citizens, but they can detain them long enough to make them miss the day's flights. Lather, rinse, repeat.

This is not hypothetical; I was relatively recently coerced this way at DUB preclearance (i.e. directly threatened with being sent to secondary with the clear implication that I would not be allowed to fly) to answer questions about my employment and whether I or other members of my family hold any other nationalities - questions that I would not have to answer at a stateside checkpoint. I did not have the luxury at the time of taking a principled stand, but neither do I plan to return to the US via DUB on future trips.

The entire purpose of Pre-Clearance is to avoid the necessity of clearance on arrival at a standard FIS. While, in very rare circumstances, CBP may choose to conduct a secondary inspection on arrival, when you arrive at your US POE, you will almost certainly be processed as any arriving domestic passenger and therefore, if connecting, will only need to pass through a TSA checkpoint if there is no airside physical route to your new departure gate. But, that would occur on a domestic connection as well.

Pretty much. For example, flew DUB-PHL. Went through DUB security, then DUB preclearence FIS security. Arrived in PHL, just walked to next gate.

Same goes for BOS if you're flying Aer Lingus or Westjet (provided you're flying jetBlue or Delta)

Air Canada, not so much... considering the gate area is the size of a cubicle and is isolated from the rest of the terminal. Thankfully, that's changing within the next 6-12 months.

That's interesting. Do you think it's possible they were bluffing and they would have let you on the plane if you had silently stood your ground?

Sure, it's possible. At the time I was on my way to a family wedding, and thus was not willing to call such a bluff or take a stand on principle over information that the US government already has from my tax filings.

If they'd demanded email or phone or social media passwords or something, the calculus might have been different.

If they were simply asking your occupation, they would also have that from the line on your passport application. CBP officers definitely have immediate access to passport application info, I had a guy who kept demanding to know if the destination address I listed on the blue form was my home address ("none of your business") and when I later FOIA'd the records of the inspection he had made notes that my destination address was different from the address I listed on my passport application.

I recommend using FOIA to get the notes of your secondary inspections, they can be quite interesting.