Basically he thinks there are two possibilities.
1. The hull itself collapsed, so the hull would be flattened, or
2. Microfissures in the hull caused water to enter between the hull and the interior metal capsule. In that case they would find the hull intact and the interior capsule crushed. This case could mean the passengers had some warning of a few seconds or minutes. I don't know where someone came up with 25 minutes, but I saw that stated in the comments.
The engineer thinks the second possibility is mroe likely. But we will have to see.
No one seems to have observed debris resembling an intact hull being offloaded from the salvage ship, but maybe there is more to come.