I think you misunderstand the problem. All radio transmitters have "harmonics" outside the main band (and outside their allocated frequency band). 5G is no different. There is a spec on how much harmonics and "spurious" RF they're allowed to generate outside the band and for the vast majority of applications this is more than enough to ensure no problems arise as the receiver of equipment designed outside the band would just reject the low "background noise". Radar altimeters however have to be (in order to function) very very sensitive and cannot easily just filter out noise from their own transmission, which makes it far easier for them to get influenced. The slightly higher margins in the EU mean that the main peaks of the harmonics lie just outside the band of radar altimeters while in the US the harmonics lie just INSIDE the band of the radar altimeters (thus causing problems) and their higher power means that they are more likely to lead to problems.
Radar altimeter manufacturers did nothing wrong when these devices were designed. RF spectrum rules as applied back then would have ensured no harmonics and spurious emissions from devices in adjacent bands would have interfered (as the frequency bands separating them would have been wider).
Thus the rule still stands, it's the 5G masts causing radio emissions outside their assigned spectrum band (or atleast increasing background noise levels) that can interfere with radar altimeters and it's thus 5G musts that must prevent that from happening.