This is only sorta-kinda true. Depending on your house and the species in question, a lot of spiders and insects you might find in it have just come in from the outside, making their way under door jams or all sorts of not completely sealed cracks and crevices. I often (relatively speaking) capture and toss out wetas in my place that I know just squeezed in from outside. It also doesn't really matter how many generations they've been there as long as it hasn't been so many that they've actually evolved to habitate in your house specifically. What matters in that case is the individual which may have been set up in such a way in your house that yeah, chucking it outside will be a death sentence. But that's really variable depending on the species in question, the conditions outside, where you end up putting it outside (example: out in the open where it will immediately get got by a predator, or in the kind of underbrush it's naturally suited for anyway), and all sorts of other things.