Zoro is and will always be closer to Luffy in strength to Sanji. Sometimes he is equal to Luffy in strength. Currently Luffy is stronger than Zoro and that is because of his awakening and haki growth. If Luffy didn't have both he would be equal in strength to Zoro.
The main power that allowed Luffy to hang with Kaido, fight evenly against Kaido, force Kaido to recognize Luffy as a worthy opponent and beat Kaido was his haki growth. Yes, DF awakening did play a role in Kaido's defeat but if it wasn't for his haki he wouldn't have beat Kaido.
Zoro, like Luffy, grew stronger thanks to his haki growth which included his ability to manipulate haki, it's strength, and size.
I know, some people might be thinking. Luffy haki growth was more and his is stronger. Well, you're wrong. Both of their haki (CoA/CoC) growth and strength is more or less equal. This can be determined by looking at Oden and understand his strength.
Oden is an incredibly strong character. One of the strongest samurais to ever from Wano and the strongest samurai Kaido has ever faced. Kaido ranked Oden in the same tier as Shanks, Roger, WB and Rocks.
Like Rogers, it was the strength/size of his haki that made him an incredibly strong character. Oden's haki almost 2 shot Kaido and scarred him.
Kaido told us haki is the strongest power in One Piece.
Zoro's haki strength/size surpassed Oden's in Wano.
Ever since Zoro received enma he has been trying to figure out the correct amount of haki to us to control enma.
During his battle with King, Zoro started to realize Oden was able to use enma easily even though he was using large amounts of haki.
That's when Zoro understood the key to controlling and mastering enma was using large amounts of haki. Zoro didn't match Oden's haki output. Instead, he surpassed Oden's haki output.
We know from Zoro's use of enma when we see smoke/steam coming form swords it means overwhelming large amounts of CoA haki was added to the swords.
When Zoro first start to master enma all 3 of his swords have smoke/steam coming from them which means he is using an overwhelming large amounts of haki with all 3 swords.
The reason for that is Zoro keeps the power between the number of swords he uses even. He doesn't like one sword to have more or less power than his other swords.
The proof Zoro is the one controlling the haki output, and not just allowing enma to take his haki can be determined by looking at his battle with King where he changes his haki out throughout the battle.
No smoke/steam from the swords.
Smoke/steam coming from swords.
Zoro only uses overwhelming large amounts of haki when he uses KoH attacks. KoH attacks are when Zoro uses overwhelming large amounts of CoA haki and ACoC haki together.
Proof Zoro has greater CoA haki output than Oden can easy be determined by the lack of smoke/steam coming from Oden's blackened CoA haki swords.
Now let's look at Zoro's CoC haki.
Not only can Zoro use his ACoC separately from KoH attacks.
Zoro can match his overwhelming large amounts of CoA haki with equally overwhelming large amounts of ACoC haki. When he does that he calls his fighting style, dragon sword style.
As you can see from my post, I proved Zoro has greater haki output than Oden. Factor in Zoro's CoA only being strong enough to scar Kaido and that proves even before his grow Zoro's haki was stronger than Oden's and has more potential. All oh that means Zoro surpassed Oden during Wano and has the power to kill Kaido.
That's explains why the author notes stated Zoro's DD attack could slay a dragon with a single hit. KoH has the power to scar Kaido. While Dragon style has the power to slay Kaido.
Zoro being stronger than Oden puts him much closer to Luffy strength than Sanji.