Which Horus Heresy book was this?
Also, Thing's weakness isn't so much flamethrowers as flamethrowers are the only way to take it down outside of armor. After all, a human is also vulnerable to a flamethrower. What if the Thing got it's appendages on some actual tech?
Nemesis
And... WH40K has armor. (Leman Russ, Rhinos, Chaos Predators, Tau Hovertanks, Eldar Hovertanks, Necron Destroyers, Ork Trukks) And more armor. (Whirlwind, Basilisk, Excorsist) And bigger armor. (Baneblade, Shadowsword, Defiler) And Titans. (Including Bio-Titans, Eldar Titans, and Ork Gargants) And Exterminatus.
Hell, except for the Necrons and Tyranids, everyone has some sort of flame weapon, often the most common support weapon for basic infantry. And for the two above, Necrons have gauss flayers (rip targets apart on the atomic level) and are made of metal, while 'nids have a variety of acidic and bio-plasma "weapons".
Also, I strongly suspect you would just make nastier Tyranids if you absorbed them with the Thing.
Honestly, when your weakness is 'Flamethrowers', you're pretty fucked in WH40K. There is an ENTIRE FACTION that basically uses ALL FLAMETHROWERS ALL THE TIME.
That and the numerous species that aren't even made of any discernible biomass. And 3rd edition crons have that fluffy lovecraftian feel to them; it's the difference between a paranoia bodyshock horror blob and a distant cosmic trigger held against your skull led by Star Gods of unknowable existence.
And the ones that ARE made of biomass are distinctively capable of defending themselves quite well.