The observation planning assumed that -1b would have an atmosphere similar to Venus. This is why they observed five eclipses, since a Venus-like atmosphere would have been just detectable after combining all that data. What ended up happening is that the dayside of the planet is much hotter than predicted for a Venus-like atmosphere, and the eclipse was actually detected right off the bat on the first observation!

Not necessarily. It could be that -1c has a Venus-like atmosphere.

For two reasons:

1. The inner-most planets are the easiest to observe. It will be almost impossible to observe e, f, and g in secondary eclipse, though there is a transmission spectrum that has been taken of -1g, and another group has observed -1c in secondary eclipse.

2. It is an overstatement to say that -1b (or any of the planets) would obviously be a bare rock in space. Indeed, the planning for this observation assumed a roughly Venus-like and cooler atmosphere that would have required all five secondary eclipse observations combined to detect any signal. In the event, the planet is a hot rock, and the eclipse was seen in just the first of those five observations, which was quite surprising!

This has changed a bit. More recent analyses of the TTVs in the system put all the Trappist planets right on the line for Earth/Venus-like composition. See Figure 12 here: https://arxiv.org/abs/2010.01074