We have different dialects too, but I don't think any of them say it like this. It's just a computer voice you know, there are bound to be some errors. Btw you can hear people from different regions say things on the site I linked to, so in some cases you can compare dialects on there (not in this case at the moment).
it is a personal pronoun like 'he' or 'she', and it's like to in Polish.
this and that are demonstrative pronouns, used to 'point' to things. In Polish, you have ten and tamten which in theory mean the same as 'this' (ten) and 'that' (tamten). But in practice, you use ten much more often than one would use this in English, and tamten much less than one would use that in English, so they don't correspond very closely. Swedish and English are much closer, so that it's reasonable to say that whenever you say this in English, it should always be den här (or det här) in Swedish (or denna/detta which means the same, we have two words for it).
In a sentence like this one, det is a placeholder pronoun which does not refer to the spiders. You use to the same way in Polish, but in Swedish, it is possible to use det här or det där instead with the same placeholder function but with a different nuance of meaning. This is possible in Swedish and in English, but as far as I understand, not in Polish where you would only use to for this. I'm learning Polish myself so I'm no expert but as far as I've understood, it's like this:
Swedish sentences with Det är/Det här är/Det där är … will all be translated with To… in Polish, but they can be more closely translated into English using It is/This is/That is …, respectively.
On the other hand, in the plural, English does not like to use it to start sentences like this. So where we will say Det/Det här/Det där är … spindlar in Swedish, in English they will have to resort to They are/These are/Those are … spiders whereas you will still happily be able to use to in Polish, and only change the verb To są pająki. This is hard to understand for many native speakers of English, but will feel more natural to you. So you win some, you lose some :)
Frankly speaking, when I learnt English at school, we were never taught to make any difference between "this is" and "that is" except maybe the first one is near and latter one is further away, but that's it. And as I see in real life, at least within the circle of English speaking people I move in, nobody seems to give a damn about that. Especially in my native language German we also have these two words, but in spoken language only one of them is used.