It's not just code coverage that matters. It's the code path selection that matters. If you have a ton of branches and you've evaluated all of them once then yeah you sure might have 100% "coverage". But you have 0% path selection coverage since a single invokation of your API might choose true branch on one statement, false branch on another statement, and a second invokation might choose false branch on the first and true on the second.

While the code was 100% tested, the scenarios were not. What happens if you have true/true or false/false? That's not tested.

There's a term for this but I forgot what it is and don't care to go spelunking to find it.