I is actually both. If it would be just a fallback then the time it uses would be at most 5s, more realistic 3s.
The think is, if you are on a internet connection where you have frequent loading time of 3s for the _basic_ content, then you are used to seeing partial loaded pages and you likely love it if the connection happen to load the content you care about first, potentially leaving the page before it's completely loaded when noticing that it's not what you are looking for.
The choice of 8s looks _a lot_ like they where looking for the highest time they can chooses without making it look obvious deliberately but which is long enough to make many "causal" users stop using the script blocker.
I.e. 10+s would look way to obvious as a intentionally long choice. 9s still looks big as we are used to see 9 as nearly 10 from product prices to some degree (e.g. 9.99€). So the highest number which doesn't look obviously high is the 8. ¯\_(ツ)_/¯
The think is, if you are on a internet connection where you have frequent loading time of 3s for the _basic_ content, then you are used to seeing partial loaded pages and you likely love it if the connection happen to load the content you care about first, potentially leaving the page before it's completely loaded when noticing that it's not what you are looking for.
The choice of 8s looks _a lot_ like they where looking for the highest time they can chooses without making it look obvious deliberately but which is long enough to make many "causal" users stop using the script blocker.
I.e. 10+s would look way to obvious as a intentionally long choice. 9s still looks big as we are used to see 9 as nearly 10 from product prices to some degree (e.g. 9.99€). So the highest number which doesn't look obviously high is the 8. ¯\_(ツ)_/¯