I think we need to step back a bit from labels like "autistic" and examine what is going on in Lalli's life, right now.
You don't have to examine what is happening right now, which is indeed outside of his everyday life, but what was happening while he was home.
He is incapable of understanding jokes (or only the jokes Tuuri says), to understand Onni's sarcasm. He did not react when hugged by Onni (neither all the times he has been hugged by Tuuri).
This implies not simply introversion, but general difficulty to recognise social cues.
In fact, we have seen him recognize another person's emotion only when their reactions were extreme. Like Tuuri hanging from the rails on the boat because there wasn't any view, Emil trembling from head to toe, and Emil lying down on the library floor.
I'll add that he did not realize that Tuuri didn't really intend to leave him alone in the train station, and usually only small children would believe such a trick.
(Also: a commenter said that each time when Lalli replied "ok" without understanding what was going on, it was probably because he has been taught that "it is appropriate to respond when talked to", and that time he raised his hand to ask a question it was most probably because he wasn't able to tell when it's ok to interrupt and when it wasn't, and he was taught to raise his hand to be polite.)
All the above signify that he has a hard time to recognize social cues. This is a characteristic of either REALLY socially inexperienced people, or autistic people, not introverts. I really don't see why he should be more inexperienced than Tuuri- they grew up in the same environment. If he was an introvert, then he would be used to HER and Onni's company, and would learn to socialize.
Another thing: he tends to do whatever is told instead of making decisions of his own. A commenter with autism said that it is much easier for her to be led in stead of making ANY kind of simple decisions. Lalli quit his job and has been doing exactly what he's told to ever since.