That’s not ambiguous though. The answer is, unambiguously, that we don’t know how many seconds there will be. But we can still agree to meet at that time, assuming we will both continue using the same time standard.
I get OPs point though - coming from an outsiders perspective I would have thought that a key feature of a time format would be to know how much time there is between two other times.
The standard for communicating time between humans lacks this feature for times more than a year in the future. That computers (which interface with humans) implement this standard faithfully is a feature, not a bug.
I agree it should be implemented but it could be implemented differently. Just like zoned datetimes are represented as a UTC time + timezone. UTC could be represented as TAI timestamp + leap seconds.