1) Xau-Tak is strongly associated with water, madness and Undeath whereas Renmark is more associated with nature and the four seasons. Furthermore, it is implied to swim 'corpse oceans' not lurk inside planetary cores.
2) Mur was freed from Renmark's core by Guthix, but recent lore from Shadow Reef implies that Xau Tak has been operating since at least the last Great Revision, especially if the Dragonkin have it featuring in their legends, as well as the ancient Infernals. Furthermore, if it is on the level of an Elder God (as it seems to be implied), would it really need Guthix's help in freeing it?
3) We shouldn't discount the existence of other threats like the Pests, the Queen of Ashes, Cthonians and other threats that lurk at the periphery of our understanding. Ol' Xau Tak isn't the only massive threat lurking out in the cosmos.
4) Finally, Xau-Tak's modus operandi is based in spreading madness and corrupting outcasts, not feeding off of it like a 'bloated parasite' as Mur is described as being. Yes, Xau Tak does have a connection with the undead but in this case it appears to be more of it being a twisted parody of life and the natural order than a result of 'feeding'.