I don't have reputations enough so for that, the most understandable answer is that they are not logia fruit users, i suspect that their are other types of devil fruit in which the body can produce as much amount of the matter(i unable to say the exact word) as they want. For example- admirals (more plasma, ice, or light), trebol( Donquixote Doflamingo family more sticky material), Blackbeard(more darkness), ace (more fire) etc. In such case the haki attack was only hits the matter covering their body and only able to scratch a little and therefore in this type of devil fruit power the body can be regained back to normal by just producing more matter. Please upvote if You find this helpful.
Obviously the admirals have logia-type fruit, check this:
Akainu fruit: Magu_Magu_no_Mi
Aokiji fruit: Hie_Hie_no_Mi
Kizaru fruit: Pika_Pika_no_Mi
The problem here was the level of the power of the haki, this case the Busoshoku haki.
You have a lot of examples of different battles where you can see that the level difference in this type of haki allows one user hit other. A clear case is the fight of Roronoa Zoro vs Pica, where Zoro manages to hit Pica thanks to that his haki is superior.
Also the admirals are it affected for this haki, this is clear when Rayleigh used Haki to stop Kizaru and even touched his Logia body.
The admirals are logia users, there isn't a fourth type of devil fruit.
They are one of the strongest, so weak punches won't hurt them that much.
The following parts will contain some spoilers for the Vegapunk arc.
We know that Aokiji and Akainu battled on Vegapunk, and Akainu came out as a winner. Akainu even lost one of his legs.
Haki users usually use armanent haki to both attack and defend themself from other attacks. You can see this in most battles between haki users, they harden their stomaches and arms to not get hit so easily.