The underlying reason why people talk about Koby and Sanji is because of CoC and it's association.
1. Parent-children association
Luffy-Dragon, Yamato-Kaido, Ace-Roger, Katakuri-Big Mom, are some examples, but it's not relevant here.
2. Captain-member/Senior-Junior association
If CoC users have a leader, the leader is CoC user. Relevant for Sanji, but unknown for Koby
3. Teacher-Student association
Zoro-Mihawk, Luffy-Ray, this thing may lead to the "Garp has it, so Koby might too".
4. Rivals/Peers association
Chinjao-Garp, Garp-Roger, Roger-Whitebeard, Kaido-Big Mom, Shanks-Mihawk, are examples. Because of this, "Zoro has it, therefore Sanji will too" appears.