The definition you provided is a simplified statement of the concept of antiparticles, but it does not capture all the nuances. The formal definition of an antiparticle is indeed more precise and takes into account various properties, including physical charges and quantum numbers.
Antiparticles are counterparts to particles with the same mass but opposite values for certain properties. These properties include electric charge, lepton number, baryon number, and other quantum numbers that describe the particle's characteristics.
In the case of the neutron and antineutron, it's true that they are not identical due to differences in baryon number. The neutron has a baryon number of +1, while the antineutron has a baryon number of -1. Baryon number is a quantum number that is conserved in particle interactions, and it distinguishes between baryons (particles made up of three quarks, like the neutron) and antibaryons (antiparticles made up of three antiquarks, like the antineutron).
So, while the neutron and antineutron have the same mass and opposite electric charges, they differ in baryon number and are not considered exact antiparticles of each other.
The formal definition of antiparticles takes into account the full set of properties, including quantum numbers, that distinguish particles and antiparticles. It is a more comprehensive definition that accounts for the complexities and differences between various particles and their antiparticles.