I recommend you reading Diels' monograph ("Droseraceae",
Pflanzenreich IV. 112. (1906)). On page 8, he writes:
"Naehere Untersuchung zeigt, dass die Zwiebel aus Blaettern
besteht, welche hochgradig miteinander verwachsen sind und
nur ihre Spitze frei herausragen lassen (Fig. 3A)."
This can be translated as: 'Closer examination discloses the
bulb to consist of leaves which are grown together to a high
degree with only their tips projecting freely'
Is this sufficient to covince you, now?
I think we can rely upon the expertise of Diels, Hooker
(_D.bulbosa_), and Morrison (_D.bulbigena_), all of which were
excellent botanists who certainly did know what they were
talking about!
It seems after the discussion we had, the only group which
could be called tuberous _Drosera_ with some justification is
Ptycnostigma (_D.cistiflora_, _D.pauciflora_, and related species).
They produce what could be called root-tubers (as opposed to
stem-tubers, e.g. potatoes).
Kind regards
Jan