Master Yi can lifesteal off Kayle while invincible?

When Kayle has her ult shield on, Master Yi is still able to lifesteal off her somehow, even though she isn't taking damage. Is this a bug?

http://boards.na.leagueoflegends.com/en/c/gameplay-balance/ATIEPVd0-why-can-onelifesteal-when-the-damage-wasnt-applied