I personally think the question is wrong here.  Not even Bitcoin Core is technically 100% compatible with the same version of itself, so asking that question is starting with a false premise.  As previously mentioned, there is a lot of work to be done in the formal verification space, but even then, due to hardware differences, compiler optimization differences, OS differences, and a myriad of other factors, I personally think it's unlikely there will ever be absolute 100% certainty.  The question then becomes, does there really have to be?

To me, what it really comes down to is probabilities.  The popular assumption is that Bitcoin Core has an extremely low probability that it will fork against itself.  So, let's go with that premise and say that the probability of Bitcoin Core forking against itself is something like 1 in a 100 trillion (1e14).

When an alternative implementation passes all consensus tests and covers all known corner cases, then it too has a relatively low probability of forking.  If that is not true, then the consensus tests need to be improved.  For the sake of argument, let's assume that such an alternative is 3 orders of magnitude, or 1000x, more likely to fork.  Well, then the forking probability relative to the Bitcoin Core forking probability previously stated would then be 1 in 100 billion (1e11).

Moreover, if/when a fork does happen, it identifies a case that the consensus tests missed and can be used to improve the consensus tests which in turn further reduces the probability for all nodes involved (including Bitcoin Core) that validate against the consensus tests.

It is estimated that in any given year the odds that you will die from an impact of an asteroid or comet are between 1 in 3,000 and 1 in 250,000.  However, that probability is still low enough that I bet you don't live in a steel box and refuse to go outside just in case.