It's got to do with the resolution, you'll come to find that the website turns into phone mode if you resize the browser down to beyond 50% or so on pc.
So half of 1920, if you've got a screen over 960px in landscape mode then you're good to go.
Then again, no phone exists with a resolution of 960x###, that's pretty much a tablet.
Androids are the best most of the time though when it comes to sites.
Yes, it is possible to post comments on phone, but it depends on the phone.
I'm no expert, but I'm just stating what I know in hopes it'll help you understand better as of to why it might be.