You are the MVP! That solved it, thank you!
db.streams.updateMany({'matching_type': {$exists: false}}, {$set: {'matching_type': 'AND'}})
{ "acknowledged" : true, "matchedCount" : 1, "modifiedCount" : 1 }
I did get an “illegal character” error first though, after copying that line. I think it was the ending back-ticks instead of single quotes, because after typing it all in manually it worked (and that’s when I noticed the back-ticks).