Having had drives which sat for many years and spun right back up without corruption makes me think 1% is too generous maybe 0.05% per year at most

The claim you're responding to is that hard drives lose "magnetic charge" at a rate of 1% per year, not that bits get corrupted at a rate of 1% per year. The error correction in hard drives is far simpler and weaker than what's used in SSDs, but it does exist. So we should expect that there's a significant margin for data degradation before any observable data corruption begins. (This is true for SSDs, too; the first symptom of data degradation is reduced read performance as slower, more complex error correction methods kick in, then much later the host starts to actually get read errors or bad data.)

The magnetic strength of particles on the disk can decay at 1% per year, but the drive won't have issues reading them until they fall below a threshold where they can no longer be read. It could take decades.