This has changed since you were here, especially in the capital..
No, they haven't. A Nigerian student who doesn't speak fluent Polish doesn't have a snowball's chance of finding a job anywhere in Poland. Even if the chances have doubled, that would mean two in a million instead of one in a million. But considering the recent flood of Ukrainians into the country, I'd bet that the chances for a Nigerian have actually gone down.
Encouraging naive, gullible and desperate third-world students to come to Poland on the extremely unlikely probability of finding employment is irresponsible and cruel. The chances are astronomically higher that they will be unable to finish their studies and will have squandered their parents' life savings in the process.
Poland is a lousy choice for third-world students who have to earn their keep, and will remain so for a very, very long time.
As for Warsaw being "cosmopolitan", far from it. Take out the Ukrainians and Vietnamese, and you're left with about 1% non-Polish residents.