While I would advise following their directions, that kind of statement is more of a "we can't possibly know all the random things out there and can only test for our target market".
In general there is no problem using a jump starter with a peak voltage that is no higher than your alternator's peak. That ensures there is no additional stress on the electrical system. This is practically never an issue with a lead acid powered jump starter pack, but if you have some generic chinese jumper make to a low price point that has too many Li-Ion cells in series, that has the potential to risk your electrical system even if you do have a lead acid battery (but for example it is not taking charge or the battery terminals are too corroded, etc so the system still sees excessive voltage).
Last edited by Dave9; 01/14/20 04:11 PM.