I am not a psychologist. I have no citations. Theories here come from personal experience and loose interpretations of things I read. Take this with a grain of salt.
When awake, you get sensory inputs: vision, hearing, etc, so as to see your environment. When asleep — when rapidly eye moving, at least — you don't, but your brain keeps running to deal with all that information.
When you listen to what people say, or read what people write, you get much more input. Sure, the raw sensory data is of about the same size, but the amount you interpret from it can be far greater. Without speech and writing, without language or other artificial mechanisms, all you learn about is your immediate, usually ordinary environment. With language, you get things like this essay, learning about samples of my experience and thinking over months in a few minutes.
In the ancestral environment, there was no language. Not much language, at least. Our current rates of artificial inputs only developed (by the spread of literacy, mass publication, and recording) in a few recent millenia, otherwise known as "instantly" (to an evolutionary psychologist, at least). So the brain is not necessarily adapted to process such dense input as it would in sleep.
We know that sleep affects learning, but there doesn't seem to be an effect the other way. You don't feel like sleeping more after getting more mental input. This doesn't mean that you don't need more mental rest after getting more information.
Input from reading or viewing other media or crafted objects is just as dense with information (perhaps more dense) as that from conversation. Conversation often elaborates — in trivial ways — on what's happening just around the people, or what they've already experienced. Written material, or crafted objects, are made with great thought and organisation. You often select such things for their unfamiliarity, and thus higher information density.
So if you're trying to be alone as a mental rest (as you probably should to function), reading or whatever as you do so breaks the effect and misses the point. This despite that you'd totally seem alone, easily avoiding any direct encounters with people. Solitude with books — solitude with tools — it's all fake solitude.
Some people (perhaps the autistic?) would experience interaction with people (obvious non-solitude) as more mentally intense, as giving more information, than the common fake-solitude alternatives. The alternatives remain fake, tho, just better than obvious non-solitude rather than probably worse.
Writing while alone sounds like "solitude with tools", but writing — if you brought nothing else — just extends your working memory and doesn't mess up your thinking with new input.
Your motive for seeking solitude might not be time for mental processing and rest, but something else. (You just don't like people, or something. I don't know.) In which case, those activities I decry as "fake" are probably fine. Just be sure to consider your goals and whether your methods towards them are actually effective.
Oh, wait, that applies to everything.