It's just putting two and two together. Between the people who admit that they have botted (if you've known they for a while), and the people who one might suspect are botting (how many people can click repetitively and not even have the slightest desire to say "Hi," despite having time to do so), what conclusion does one draw?
If you are metagaming you do not have time to do so. Typing takes precise ticks which impacts on the xp:hr in the long run.
Plus being unsociable is hardly a good measure to go by at all many people be clicking away but watching tv or talking on skype and not paying attention to chat, some will even have chat turned off.
Not talking is the absolutely WORST measure of botting, especially seeing as most bots these days are more than capable of holding a basic conversation so if anything passing basic chitchat of hi, what's your level etc. is more indicative of a possible bot than not talking at all.
I remember when bots used to talk. I would say "Stupid level?" And much to people's bemusement, the air altar became filled with quick-chatted "My Runecrafting level is ." And if you try out some of those AI programmed to hold a conversation, you'll see it's easy to distinguish a difference after a few lines-they tend to say something random or miss anything subtle.