That's the point, in the real world it's called deuterium burning but because we're pretending that deuterium oxide (which is simply D2O aka heavy water) is deuteride and its ideal neutron chain reaction is somehow fusion (without creating fusion products), it becomes kind of silly to say it's burning. Let's face it: deuterium is just a hydrogen isotope so let's not get caught up in names.
Burning isn't the right word. Deuterium is liquid and it isn't flammable. Just say compressing.
You're right in the sense that in the real world deuterium burning (into helium-3) is fusion, but in TPT it's the element hydrogen that fuses into noble gas (the closest thing to helium in TPT). Deuterium in TPT is a neutron/proton multiplier that leaves neither fusion nor fission products. +1 For the realism plus the fact that your primary functions pretty reliably despite its small size.
Now before you say "deut no fusion -11111111111" read that.
Thermonuclear bombs are where a primary fission core (When enough force is added to an unstable element to make it split causing extreme energy) to ionise an element (the most common for the US millitary is FOGBANK) that then applies the heat (around fifteen million degrees celcius, 3 times hotter than the sun) to fuse deuterium-tritium (or lithium-6 deuteride or pure deuterium) causing a more powerful explosion.
Before anyone says it, this is fusion. A breif description of thermonuclear bombs and nuclear fusion is in order. Right. Nuclear fusion is when things get enough heat (or, like the Sun, heat and pressure) that the atoms of them fuse. Hydrogen is the most common fusion element so they use isotopes, deuterium (a proton ((a hydrogen atom)) and a neutron) or tritium (proton+2 neutrons).