The "truthers" own report claims that collapse due to fire was "improbable" because according to their estimation it would have required simultaneous heating of all support columns.
However, according to experts' report(s) (i.e. Preliminary modelling of Plasco Tower collapse) (2018) what happened was that the reinforced concrete floors buckled under the heat and internally fell one on top of the other. And this caused the columns to then collapse inwards. As far as I can tell the "truthers" report didn't take this possibility into account, while other works evaluated it fairly thoroughly through simulations etc.
There's also more recent (2020) paper (by different authors) but having the same general conclusion of how the collapse proceeded (i.e. floors first). This analysis also found that connection failure between beams (supporting the floors) and the columns could have occurred due to the heat (of the fire):
An important observation in debris was the out-of-plane bending of the beam-to-column gusset plates. Based on the results of numerical analysis, this phenomenon has occurred because the filler plate of the top and bottom chords in the main trusses was present only on one side of the gusset plate. This arrangement creates an initial imperfection and consequently an eccentricity in axial force of chords, which develops an additional out-of-plane bending in gusset plate under gravity loads. As the deflection of the beam gets larger at elevated temperatures, this out-of-plane moment becomes more significant, which may eventually lead to connection failure. As indicated in Figure 19, initially, von Mises stress of diagonal bars and welds of the end angles reach to maximum tensile stress, and then von Mises stress of other members exceeds the yield stress of steel material. Debris observation in Figure 19 also shows this failure well.
More generally, somewhat subtle design flaws in joints (that e.g. only become an issue when seriously heated) were probalby fairly hard to analyze in the 1960s when that building was designed. There are e.g. some examples of US truss bridges (built in the same decade) collapsing due to connection design flaws (and no heating was required, but there was extra load and some rusting involved).
Nowadays truss systems (for use in buildings rather than bridges) are more serious scrutinized how they behave under fire conditions, at least in the US. (Wood-to-metal truss systems in particular can collapse in mere minutes due to differential heating of the materials and charring around the gusset plates.) Even for full steel truss systems, US standards of the 1980s only required they withstand the fire for 2-3 hours, which suggests that it's probably difficult to economically engineer them for more. It was also noted at the time that
practical fire tests were difficult due to the need of large furnaces (presumably computer simulations were still rather impractical/expensive) so only joint sub-assemblies were tested in this concrete way. Issues like the effects of whole beam elongation would probably not have been accounted for except by calculation (p. 8).