He's not. He's scoring at a clear level above them.
Non penalty goals per 90 in their Serie A seasons (including current seasons).
Lukaku: 0.51, 0.56, 0.26
Tammy: 0.41, 0.31
Osimhen: 0.57, 0.64, 0.83
In other words Osimhen's worst season in Serie A is better than any of Lukaku or Abraham's in terms of goal rate. And we see the same same thing when we look at their underlying stats.
Lukaku: 0.43, 0.57, 0.33
Tammy: 0.49, 0.55
Osimhen: 0.61, 0.62, 0.59
Other than having played in the same league, these are not comparable players.