You are now my favourite lizard ever.
Where was your head previously?
As noted, the Seraphon are formed of star stuff, summoned into corporal form by the vast intellects of the Slann Starmasters.
"Like arrows of light falling from the heavens, the Starmasters arrive upon the battlefields of the realms, summoning their Seraphon armies from distant memory through sheer magical might."..."then as swiftly as they arrived, the Seraphon vanish once more into the stars." (4).
"After the death cries of their enemies fade into silence, the Seraphon depart, leaving eerily quiet battlefields behind, littered with the broken bones of those that presumed to stand against them." (6)
The first battle where Seraphon and Stormcast fought together in the Gorevale is described. At the end "the Stormcast Eternals watched as their seeming allies returned to the stars. Sigmarite hammers were held high in salute as the Seraphon vanished in beams of starlight. Neither were to know that many more foes would soon face the combined might of Stormcast and Seraphon" (15)
No realmgates needed here. And the specifics of what happens when a Seraphon dies is explained with reference to the Saurus Eternity Wardens who "have lived the equivalent of many mortal lifetimes in the service of the Slann, suffering repeated destruction within the Mortal Realms so their lord might live, and returning from the stars to do so again and again.
Should an eternity warden fall a Slann can summon his protector back regardless of the manner of his demise, or the presence of barriers either mundane or magical."
The seeming ease of summoning, the lack of a time consuming and unpleasant reforging process, and the simplicity of re-summoning after death really reduces the stakes for an individual Seraphon when you compare it to all the hassles faced by Sigmar's chosen.
I would liken the gravity of the whole Seraphon getting killed situation to this: