sustained loud noise disables some participants microphones

Zoom does noise control at several stages of audio processing.  If you had an extremely loud ongoing sound, my guess is that Zoom's noise limiters acted to suppress the loudness of the sound being sent by your computer (and possibly also from the Zoom server), and once that noise limiter reached the fullest suppression point, it takes a few seconds to "recover" as it measures the new "reasonable" sound level and begins the process of easing the limit off.

Not sure if your meeting depends on the loudness of an actual airhorn... but you might consider recording the horn with your recording volume level adjusted pretty low and editing the sound file so that the loudness level meets your needs, then play the sound through your desktop audio or something.  There are a number of techniques if you're willing to work at it a bit to make it work better overall.

Consider opening a meeting with a couple of attendees, turn on the recording, talk, sound the horn, ask others to speak a few seconds, then turn off the recording... then upload just the audio file (or post a link to it on DropBox, etc.), and I'll take a look at it with a couple of audio tools I've got.

Original Sound  on could make a big difference. The expectation by the Zoom client is that you will condition the audio input to the appropriate level. Let me know what you find!