It's worrying that seemingly only Americans don't know what he meant.
The NBA is a national league with two countries, one of which has only one team. It doesn't make sense to title yourself world champion if your competition is two countries.
It makes as much sense as Miss Texas calling herself Miss Universe because she beat the other states in a Miss America beauty pageant.
If after the season, the Celtics played in the intercontinental cup, they'd be world champions
en.m.wikipedia.org
But the NBA decided to send the g league winners instead. Ironically, if they win the cup they can call themselves world champions. But they didn't. No G League team has beaten international clubs