Does this mean I can't find people on here from other platforms?
Not seeing what you're looking for? For reasons pertaining to anti-stalking and harassment, search scope only includes usernames, hashtags and posts you have made, were mentioned in, or interacted with.
@Nick_Searles If you know their handle or display name you can, but if you're searching for content of posts, only posts you made or have interacted with will show in the search for you.
This helps prevent folks nefariously searching for somebody saying something 'bad' so they can screenshot it and use it against them.