If you agree that the Abhidhamma is part of the Pali Canon and if you think that the Abhidhammattha Sangaha describes the Abhidhamma well and if you think the introduction and guides and notes of Bhikkhu Bodhi to it is strict according Theravada orthodoxy, then I use a quote of it:
"According to the Abhidhamma philosophy, there are two kind of realities - the conventional (sammuti) and the ultimate (paramattha). Conventional realities are the referents of ordinary conceptual thought (pannatti) … The Abhidhamma philosophy maintains that these notions do not possess ultimate validity, for the objects which they signify do not exist in their own right as irreducible realities. …
Ultimate realities, in contrast, are things that exist by reason of their own intrinsic nature (sabhava). These are the dhammas: the final, irreducible components of existence .." (p. 25)
This is not Bhikkhu Bodhi's private opinion but a description of a deep rooted (mainly in Burma) view on the two truths according Abhidhamma.
As far as I know this two truth idea does not occur in Abhidhamma texts themselves but were developed later; perhaps in the same period when in Mahayana the two truths ideas were developed.
If you however think this is not what the Buddha teached, that this is essentialism and an ontological interpretation of the Dhamma that however had to be understood in a soteriological and phenemenological way (Nyanaponika Thera), then I agree with you.
The for a Theravadin most usefull translation and introduction to Nagarjuna's 'Philosophy of the Middle Way' is of the (Sri Lankan born) David Kalupahana. To him this text is a commentary on the Kaccayanagotta Sutta, with its core;
" 'Everythings exists,' - this, Kaccayana, is one extreme
'Everything does not exist,' - this, Kaccayana is the second extreme
Kaccayana, without approaching either extreme, the Tathagata teaches
you a doctrine by the middle." (SN 12:15)