I have personally never cut into my duck's foot. And you are right to cautious doing it. Anytime opening up a wound you do have a potential for causing an infection. Especially for that injury, it hardly looks like anything to really be worried about. But honestly, you have to do what you feel is best.
If it were my duck, I treat bumblefoot with enrofloxacin (baytril) for bumblefoot to kill any infection inside. Enrofloxacin is one of the most effective antibiotic for bacterial infections. It does has it potential downsides, as the FDA does want people using it on poultry anymore. Certain amounts of antibiotics end up in the eggs, so it as has the potential to led to antibiotic resistance bacteria. However, a month after the last dose for most antibiotics, you wont find them in eggs anymore. And at this time in the year, most ducks have started to stop laying for the year. So as long as you don't eat the eggs and follow through on the full treatment, the risk is low.
You can treat it on the surface with bacitracin found at any local pharmacy. I have also found from people in other countries that don't have access too topical antiseptics, that medicinal honey works quite well. A lot of people treat minor bumblefoot issues this way without any antibiotic.
From my experience with bumblefoot, I am uncertain that such a minor bumble would necessarily such a limping issues. Its not impossible, but just to fully cover your basis, I would fully examined the leg. I would compare it to the other one and feel for any differences. Any noticeable swelling or warming on the rest of the leg?