Just played a ranked match with a lvl 21 LB how is that possible?

1. You didn't play ranked
2. Whatever you used to check his/her level wasn't providing the correct data

If you chech her account on op.gg she was level 30, probably a bug.

She also has a page with 30 points in masteries.

OP.GG is not accurate when it comes to sub30s.

You cannot trust information on third party websites. Period.

ProphecyOfBonds is level 30. The LeBlanc you just played against.