When you are discussing the ability of a pet to tank - then yes - enmity is definitely a relevant factor and not an irrelevant tangent.
It's typically irrelevant in the context of how we're tanking. In SMN's case,typically you're only tanking against other SMNs, people who know what's going on and control their output accordingly, etc. PUP can generate plenty enough enmity with the release of the 2nd strobe attachment (and keeps itself alive with repairkit regen, repair oils, and also cannibal blade healing it for like 1k and never missing). The only difference with BST is it doesn't have to do anything to tank. The reason it's an irrelevant tangent is because it's irrelevant to the fact that tanking on the other pet jobs (except DRG) is possible.

In shorter terms, the simple fact is PUP can tank when geared properly. SMN only ever has to tank against itself and/or probably other SMNs, and is perfectly able to do so.