"Not just"? Let's do a thought experiment. Take two people, and let them come to a random Silicon Valley Company - Twitter, Facebook, Google, whatever, or to a college campus - and publish on a company public forum, or campus forum, etc. - a slightly controversial article - one with some left-to-the-center bent and one with right-of-the-center one. Which person would have a higher chance to earn the vitriolic scorn of the surrounding public, face HR complaints and ultimately may get fired?
Did you ever read the political spectrum distribution on university faculties? In some places, there's no single person that is not ideologically left on the whole faculty staff. In others, you get ratios like 1:10 to 1:20. This is where the students spend their formative years.
I am assuming for now that the political spectrum is one-dimensional, which is obviously wrong. But if we keep this simplifying assumption for a minute, and consider how one can form a bubble, "not just Republicans" would be quite an understatement.
For the non-lefty person in most tech, academic, entertainment, press and many other areas, there would be literally no way to live in a Republican bubble - there would be no material to form the bubble from. Of course, there are such bubbles too, and places where you could comfortably form them - but I think I would not be too wrong if I say at least 90% of HN readers are not in those places. You can form your own bubble of course by carefully selecting what you read, who you listen to, etc. - but even then if you're in one of the places enumerated above in the US, you'd encounter people who hold views different from yours - and are very open, outspoken and sometimes forceful about them - literally every day.
The reason why the right wants their own apps is not because they don't want to ever hear from the left - though some do, of course, but not all - it's because they feel the left does not want to hear from them, and is actively pursuing silencing them. And they have a very good evidence to show for it.