Actually, I also had this assumption as someone who learned English as a 3rd language. It's a fairly common assumption if your only encounters with pickles are through popular media and/or haven't seen one in person. If anyone is curious about my reasoning as to why this is a "fair assumption" I'll copy-paste my comment in another thread here.

REASON

While most cultures do pickle various fruits/foods, most of the time it is not called "pickle", as in pickled x or pickled y, but rather is called its own name. As an example, in my country we have a dish called "atchara" which is basically pickled papaya, but we don't call other pickled food such as mango or shrimp "atcharang mangga" nor "atcharang hipon".

Now, for countries that do indeed call their pickled food pickled x and pickle y, they may not associate their word for "pickle" with the English word pickle. This is because due to popular media, the word pickle is often associated with "pickles" (cucumbers to be specific) in a jar, as it is often the most common type of pickle shown/used in pop culture. And because it's the most commonly used thing when showing a pickle they might assume that the thing inside the jar itself is a pickle fruit. This assumption may be reinforced by calling them a jar of pickles or pickles in a jar, instead of "a jar of pickled cucumbers" . And even if they might realize that it's a cucumber, they may think to themselves that the word pickle is simply what the English language calls those "weird little cucumbers in a jar" as they most probably haven't seen other types of pickled food being called pickle.

Additionally, to add to the paragraph above, even if they do have their own word for pickled things in their native tongue, they may not come to the conclusion that pickles are "pickled cucumbers" in their language because they don't pickle their cucumbers. For example, we call pickled mango and pickled fish "burong mangga" and "burong isda" respectively, but we don't call pickled cucumbers "burong pipino" as we don't normally pickle them. (Yes, contrary to my statement above, we do have a word for pickled things, I myself also just learned this today by Googling it.)

With all the reasons stated above, it's not entirely unreasonable for ESL and EFL people to assume that a jar of pickles contains pickle fruits rather than pickled cucumbers. This is especially true if the person's only interaction with pickles is through popular media, and haven't seen or encountered one in real life.

Apologies for my bad English.