The only thing to know for sure is that you followed protocol, and he didn't. If you had swerved as he passed he would have held at least part of the blame for any collision, at least in my opinion.

We could speculate all day on why, but honestly, my guess would be that he is one (of many) on MUPs that simply doesn't waste the breath to be polite.

However, just to add some other possibilities to the mix:

He is mute.
He was too busy trying to not sound like he was out of breath.
He was indeed offended at being passed by a hybrid.
He didn't hear you, and felt like giving you a taste of your own medicine.
He is baffled about why people say "On your left" when they pass him, thinking that nobody is further left politically than he is.
He is way to the right politically, but has had too many close calls when he announced "On your right" as he passed other riders.
He did announce his presence, but tried not to startle you, and said it too quietly.