According to the image, it is not even a text particle, just pixel art by placing a square particle (properly a text this time) at the place where a pixel would appear. Real text particles work like (actually they are equivalent to) nametags. Nametags are always properly rotated in the UI screen. The text in the image is obviously not a nametag.
How do I "double check" I wasn't wrong after all; just that I haven't played Minecraft too long I can't even determine the appropriate size of a nametag at that distance (I can't even determine the distance because I completely forgot how high one block is).
I can tell that that is one text particle, so I'll assume that they're using a text particle, and some non-English characters. It's in Unicode, however. I found a couple characters in the text in this list. How can you tell that they used an invisible entity? Are we missing something? If so, please tell us how you concluded that they were using an invisible entity. Then you better start playing some Minecraft again. The text particle appears to have been spawned near the wooden fence above the zombie. @SOFe doesn't appear to be trying to start a fight, you are. What you've posted would've gotten this thread locked, not what he posted. He was only explaining the difference between a nametag and a text particle.
I never installed Minecraft on my phone Maybe I just saw people playing it and played it on other devices. Yes: Let's not start a fight about this.