Well, it seems significant that you could potentially need 30+ DTR to show an increase of +1 damage on a DoT,
You need random(0..30) DTR to show the first +1 and then +"30"-ish for every +1 after that.

In addition, you have multiple DoTs with different potencies. Each would have a different cutoff for breaching truncation thresholds. As a result you would see incremental increases at smaller intervals of DTR. For example, every 5-15 DTR you'd see one of your DoTs increase in tick damage.

any minor optimization of secondary stats would essentially result in no change to DoT damage.
The stat weights you referenced lead to a major optimization of secondary stats away from DTR, on the order of ~60-120 depending on SMN itemization.

That's like 2-6 damage per DoT tick.

if the damage add from DTR is being spread over the full course of a DoT [...] that alone makes DTR pretty weak on DoTs, and especially weak on longer DoTs.
Not directly. Only from potential truncation of individual ticks.

Either way the statement is false: it's not "especially weak" on "longer" DoTs. It's possibly more difficult and less reliable to itemize for on DoTs with low potency per tick.