Yes most likely most of the M1's "secrets" are know by many. However, the versions we give/sell to many of those nations are not the most "updated", etc.
Yes Chobham armor was TS when I was in the ARMY. As Bde Maint Officer, '86-'87. If the armor was damaged some how causing the skin to rip and show what was inside. We had to cover it up with a tarp, poncho, etc. And a special repair team would be sent from waaay up the Chain of Command. To repair it.
Even when ERA was adopted we with a TS had a classified briefing. You don't want to make it any easier for your enemies to get any intel about anything for obvious reasons.
Even footage of Saddam gassing the Kurds during the Iran-Iraq War was classified TS. And we with a TS viewed it in a "safe" location in Bde HQ. The wanted us to know the horrible effects of, IIRC, VX. As learning tool and to take NBC defense seriously, etc. Now of course has been shown in the media many times. Nothing new there.
You may be missing the point about the M1 I think. It is not that they do or don't know everything about the M1 or at least most of it. The fact that the Iraqis abandoned 42 of them a ran. Along with 52 M109 155mm SPFA. Many of those AFVs were not even captured but stripped of MGs, etc. And AFAIK destroyed.
It is not unrealistic to think some of those M1s were not destroyed and used by whoever. But they wouldn't be running very long as those that captured them did not have the assets, skills etc. to maintain them. As well as ammo resupply, etc. Most of ISIS et al have low skills other than using SAs, IEDs, etc.
And of course their predilection for rape, torture, bloody executions in all forms, etc., does not take a high IQ or skill set.
Regardless we saw they were using mostly Russian equipment they took from the Iraqis, etc. How long those AFVs, etc. were running is questionable? I did see even a news footage of ISIS using a US M1117 "Armored Car". link
Mostly likely taken from the Iraqis. Plus even reports of Iranians that were supporting the Iraqi Shia militias using M1s.
Again the Iraqi military like many in the region were not first class soldiers, etc.
That being said, we didn't expect them to abandon their equipment and run either. But again that should be no real surprise I guess …