Spider infestations are a very binary problem: they are resolved (0 living spiders) or they are not resolved (>0 spiders). Which one is it? So there is greater zero spiders? Then it's not resolved, Dave! Go take that sign down!
Tbf, that depends on how you define "infestation".
Maybe to them, it's a matter of numbers and behaviour, and just one or two spiders chilling in the corner is no big deal.
Maybe it doesn't become an infestation until there's 10+ spiders, crawling at various points on and suspended from the ceiling, on the walls, under desks, behind your phone as you read this...
As an Australian, I'd also argue that an infestation depends on the species of spider, and how far out of the city you are.
10 daddy long legs (cellar spiders) is a bad time if you have to walk through them, but it's not an infestation, I think I'd need 20 daddy long legs and a few hundred little babies before I say something is infested.
1 red back and an unhatched egg web would count as an infestation for me because I'm currently living in the city, but growing up in a regional town, you'd need 5+ before it's infested.
I've got about 8 chubby bum garden spiders living under the capping of the colorbond, but the fence isn't infested because they're just garden spiders and they're in the garden. That's just where they live. I feel like I'd need 50+ spiders on the fence before I consider breaking out the mortein.
This is the problem with when scientists declare a think extinct. They can't prove it's gone. They can sample and say "we haven't seen one in a while, we think they're gone".
It's the same with spider infestation. I'd you have seen one for a while, you can declare it's resolved, but you're never really sure.
PS spiders are better than bedbugs. A former employer I was at had a bedbug problem. That sucked for people in that office.