First off, Kaguya was never killed. Not by Naruto, Sasuke, Hagoromo, nor Hamura. and DEFINITELY not by Sakura.
Kaguya was split into 2. One chakra, One Vessel.
Naruto and Sasuke never got the seal because Hagoromo took em back as soon as they were done sealing Kaguya.
This is perhaps because he and Hamura can use the seals to get back into the real world.
Boruto still has his.
It could mean anything. We just know that space vampires can place seals on other people to give them their powers (possibly to take over their body much like Orochimaru had planned and done during Part 1)