The most reasonable is marco already settled in an island as inactive pirate unlike Shanks...
The second is the existence of the plot. Shanks and Luffy might be meet as Legit Yonkou. There is a possibility of propaganda after shanks meet luffy...
Luffy has 2.5-4B bounty, then Shanks will play loser/death (but actually not, shanks show CoC way to Luffy so Luffy has a knowledge to be better than shanks because of haki alone VoAT.. & E.g beckman to zoro), then one of the crew beaten purposely to go to lead the newspaper to show that SH beat Shanks. So the world know SH is the PK candidate, right above BB who the only challenger_not kidd without his influence)
Yes, shanks is snitch...