The parent comment's suggestion was: (1) for each possible input program, (1.a) compile it, and (1.b) check if the result equals the given compiled code.
Agreed, steps (1.a) and (1.b) terminate deterministically (for a given compiler).
However, the search space for this search procedure is infinite.
Similarly, it would be impossible, in general, to exhaustively test every possible input of a compiler.
And while not strictly related to "is it decidable", in the real world we have to keep in mind the complexity of the solution. The presence of the naive solution for a particular language doesn't say much about how well we can* actually do it. Is the problem actually decidable within the confines of the physical observable universe for real world inputs?
If the compiler did not halt, then we don't have anything to disassemble in the first place.
Formally, it is obvious that a diagonalization-based search will eventually find a correct input, regardless of the halting status of any given input. In practice, none of this matters very much.
You are however correct that this can be circumvented with diagonalization.