Potential isn't the ability to use the Force, lol. Potential is the capacity to develope and reach something higher. You don't have the [natural] ability to use something unless you've already reached a certain point where you can use it. Assuming it's only referring to potential, then Yaddle doesn't yet have the ability to use the Force in a way that's comparable to Yoda, but that's what the quote is implying.
The fact that endowment(an ability or quality of someone) is synonymous with affinity, the fact that aptitude means a natural talent at a certain ability and the fact that affinity was actually defined as a natural ability to use something(in this case the Force) means that it refers to actualized power.
Or we can just assume that affinity in this case means an empathy/unity/understanding/kinship and just say Yaddle's understanding of the Force is on Yoda's level, but her power isn't. I'm fine with whatever, really.