With all the anti cavalry in the game currently, the fact that Armor Emblem is incredibly dominant in the 710+ score range, and the presence of so many great Enemy Phase seals (the two Defences, Ploys, Quick Riposte), the meta has long since moved from one shot.

Plus, you seem to be underestimating Sigurd's tankiness. He doesn't survive a single hit, he completely absorbs it. Mages barely threaten him, and with double Close Defence, he's impervious to literally every physical unit who isn't Ridersbane Effie. With +Spd IV, he sits at 35/35 offences, and his Bonfires proc for over 20 on Enemy Phase. He even scores higher in Arena than Ike and Ryoma, making him great for mixed teams.

Rating Sigurd the same as Leo is completely ridiculous, and (along with Felicia and Micaiah's rather low placements), goes to show this tier list favors offensive power waay too much.