It says easily fight a Admiral you idiot. They said nothing about equally.
The only person they said Shanks could fight equally with is Whitebeard
: Despite having no devil fruit he's capable of fighting on Marine admiral level
Doesn't say he would win or loose or if it was be difficult or not. Actually the fact they say this about shanks while boasting his bounty and not the other yonko sorta implies shanks is the only one capable of said feat