Heinz Peter Bredow wrote-
"Contrary to one hypothesis, because of the primitive state of the obturator
notch of the pubis in Siamotyrannus, tyrannosaurids cannot be derived from
allosaurids, in which this character is more advanced. However, this would
not preclude derivation from sinraptorids, in which the obturator notch is
So here Siamotyrannus is described both as a tyrannosaurid and a possible
sinraptorid. This would of course mean that Tyrannosauridae would be the same
as Neotetanurae (Allosauroidea + Coelurosauria).
No, Buffetaut et al. are saying that allosaurids are too derived to be ancestral to tyrannosauroids, as their absence of an obturator notch is more advanced than the latter. However sinraptorids have obturator notches, so they could hypothetically still be tyrannosauroid ancestors.
It's a horrible arguement though, that harkens back to Heilmann saying birds cannot be derived from dinosaurs because the latter lack clavicles. Reversals happen in evolution, so you cannot use such statements to prove phylogenetic hypotheses. So they are still saying Siamotyrannus is a tyrannosauroid, just stating that the latter clade could be derived from sinraptorids. Luckily, Tyrannosauroidea is a stem-based group that doesn't include Siamotyrannus in its definition, so we don't have to worry about being a synonym of Neotetanurae/Avetheropoda.