Here's another translation of that Seneca quote, starting a lttle earlier in the letter (LXVI.8-9 )
"Therefore the power and the greatness of virtue cannot rise to greater heights, because increase is denied to that which is superlatively great. You will find nothing straighter than the straight, nothing truer than the truth, and nothing more temperate than that which is temperate. 9. Every virtue is limitless; for limits depend upon definite measurements. Constancy cannot advance further, any more than fidelity, or truthfulness, or loyalty. What can be added to that which is perfect? Nothing otherwise that was not perfect to which something has been added. Nor can anything be added to virtue, either, for if anything can be added thereto, it must have contained a defect. Honour, also, permits of no addition; for it is honourable because of the very qualities which I have mentioned.[5] What then? Do you think that propriety, justice, lawfulness, do not also belong to the same type, and that they are kept within fixed limits? The ability to increase is proof that a thing is still imperfect."
Seneca is saying that a virtue is limitless, it is infinite in that nothing can be added to it *because* it has no limits. If something could be added, it wouldn't be infinite. It seems to me it's the "adding" part that is important. Epicurus comes along and says pleasure has a limit (the removal of all pain) but, by definition, once all pain is removed and pleasure is complete, no more pleasure can be added. Therefore, as Senea says "the power and the greatness of virtue cannot rise to greater heights, because increase is denied to that which is superlatively great. You will find nothing straighter than the straight,..." Epicurus answers that by saying pleasure cannot rise to greater heights than the absence of all pain, therefore, pleasure cannot be added to once it has replaced all pain.