From engineering perspective, the 5G equipment is not staying in its band, not radar altimeters. Remember that the regs for band usage are on transmit, not receive, and the physical reality makes radio altimeters especially hard to filter.
Many receivers can be affected by strong signals outside of their band. The strange analog world has effects like desensitization and intermodulation, which can produce in-band signals or attenuate the ones you intended to hear. The degree to which a receiver is immune to these effects is part of the engineering of the receiver. They likely didn’t consider strong C-band transmissions with a high duty cycle in their design at the time.
I don't think you understand FCC rules about communications at all. You can't just freely snoop on someone's cell phone call because you are just receiving. There are all kinds of regulations around reception.
