That has not been my experience with my Battery Tender Plus+ units.Unfortunately, the Deltran Battery Tender or Battery Tender Plus are not the best products to use. The problem with the Battery Tender Plus is that it reads the current battery voltage when it is first connected and thereafter will only maintain the battery at that charge level. My friend and I both independently confirmed this through practice and called the company to confirm it as well. I had a Battery Tender Plus and a Yuasa battery maintainer and the Yuasa did a better job. I replaced the Bettery Tender with an Optimate 4, which will actually attempt to repair a sulfated battery. Both of these products will fully charge the battery.
Letting a battery sit unused for a long period of time results in it being severely discharged. Even usaing the vehicle once a month can extend the battery's life.
As long as the battery has 'some' charge, I've always been able to bring it back to full charge using one.
OTOH, with a completely dead battery it doesn't think it is connected -- so does nothing.
In that situation a plain ole trickle charger can be used to get 'some' charge into it.
This is the situation that was discussed at the start of this thread - use a trickle charger or good battery to get the 'dead' patient to have enough voltage for the tender to take over.
FWIW, I've had exactly the same issue with much larger batteries (not motorcycle / lawn mower / atv) and much larger chargers -- some of the new automatic chargers need to have a least 'some' charge/voltage on the battery before the charger will start the recharge process. An older charger -- the type that shouldn't be left on once the battery is charged -- can be used to get an initial partial charge.