Because doffy isnt the second strongest.
he was in original Warlords , but only because Moria got weaker .
Kaido has had terrible track records in gauging power level though , he expected Jack to retrieve Doffy from a Marine fleet containing Fuji Sengoku Tsuru , he also send Jack to capture Luffy and Law
Kaido saying "mere Warlords" in 923 could also be another terrible power level gauging record , as Warlords included Mihawk . in this chapter he seems to think Warlords were a force to be reckoned with though , which shows his inconsistency
lastly i strongly believe Warlords and their crews were meant to counterbalance 1 Emperor crew , and Doffy was like the 3rd strongest in every set , taking into account prime Moria
Mihawk -> Emperor
prime Moria/Yami Teach/Weevil -> YC1
Doflamingo and prime Kuma -> YC2 and 3
Jinbe-Crocodile-Hancock/Hancock-Law -> Snack/Orochi , and some Veterans/F6
Vergo Pica Diamante Trebol -> the rest of Veterans/F6
Oars/Hajrudin-Stansen-Goldberg-Rodo-Gerd -> fused Charlotte Decuplets/Numbers
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so Kaido calling Doffy weak means nothing really , as he is a dumbass .