It's not, you can send messages on the website on mobile.
Last time I tried, I was given a "download the app to use this feature" screen.
I have literally never logged into LinkedIn on my mobile. I stay as far away as possible. On the desktop - yes, occasionally. The messages are usually spam from recruiters.
I just don't use LinkedIn on mobile, and view anyone who wants to connect only via LinkedIn as generally not useful. I think it's a good sheep filter.
(I'm also not a career developer, so maybe my priorities are different.)
That's weird, I am able to message on my phone through the linkedin.com page.
For me, if I click into a post it dims grey and refuses to do anything. It's very user-hostile.