For the most part, yes. The exception is if either the instance you’re trying to post to, or the instance you’re on, have defederated (“block”) from one another then that won’t work.
There are some exceptions though. Your instance might have chosen to defederate with another one (or vice versa). Then you won’t see that content and they won’t see yours (sort of, there are some differences between defederating and being defederated, but that’s the gist).
Can I post here from different instance?
Only if you ask nicely.
You just did!
So I only ever need an account with one instance to browse the whole “fedeverse”?
For the most part, yes. The exception is if either the instance you’re trying to post to, or the instance you’re on, have defederated (“block”) from one another then that won’t work.
Pretty much. Isn’t it wonderful?
There are some exceptions though. Your instance might have chosen to defederate with another one (or vice versa). Then you won’t see that content and they won’t see yours (sort of, there are some differences between defederating and being defederated, but that’s the gist).