Not true or you are an outlier. HR on bike is always lower than running at the same RPE. You don't have propulsion and impact to deal with. Theres a reason you can ride for hours and hours on end but you cant run train the same way.
I agree. I would have to work insanely hard on the bike to have the same HR as my threshold runs.
There's some truth the newer cyclists not having the leg strength to be able to get their heart rate up but even really good triathletes will have trouble getting their heart rate on the bike close to where they can get it to on the run. There's some balance here. A runner that is newer to cycling/maybe using it as a form of cross training is going to have a bigger gap than a well trained triathlete/cyclist. But even pro triathletes are going to struggle getting their heart rate up on the bike in the same way. Their heart rate zones will still be lower on the bike. I was flat-out for 10min (almost 370 watt avg.) at the end of a 1hr sweet spot build & my HR peaked at 175. It's just different even if you're in good shape. That's what I would race a half marathon around.