I think the emphasis should always be on what effects of a drug are
appreciable - many drugs are very promiscuous and bind to many targets. The important question is figuring out which targets actually produce the desirable effects of a drug.
As Kaleida has pointed out there is certainly appreciable NRI, and NMDA antagonism is known to induce release of monoamines which could act synergystically to produce stimulation.
Between the reuptake inhibition properties and NMDA antagonism properties, I think there is a sufficient explanation of the stimulating effects and we don't necessarily need to invoke agonist actions at adrenergic receptors to explain the stimulating effects.
Regarding table 1 of the paper you posted (
http://www.sciencedirect.com/science/article/pii/S0163725816300560#t0005)
It looks like action at alpha adrenergic receptors by DXM/DXO isn't very appreciable, and the measurement of 60% inhibition at 1000nM could even be a little bit off (depending on whether an antagonist or agonist radioligand is used, measured affinities can vary, and lord knows there are plenty of issues with radioligand binding studies).
That is also just affinity - which has little to say on the matter of the intrinsic efficacy of a ligand, meaning that some ligands can bind with very high affinity to a receptor but they'll just block it and won't activate it (antagonists). So they would have to do some sort of functional assay to determine that DXM and friends are actually acting as agonists/antagonists at a2 receptors
Which brings us to another matter with a2 though - a2 receptors are actually mostly presynaptic autoreceptors that act to homeostatically inhibit the release of NE/5-HT when activated. There are some post-synaptic receptors, but for the most part when somebody takes something like clonidine (a2 agonist) they are going to be the exact opposite of stimulated.
DXM would need to be an antagonist at a2 in order for that to contribute to its stimulating properties, and by all accounts yohimbine et cetera is not very enjoyable.