Translation:This Esperanto speaker does not speak English.
"Parolas" has the present tense verb suffix, so "ne parolas" means "does not speak/is not speaking". It does not define cause. You're absolutely correct that context would be needed to further explain the situation!
"Tio ĉi esperantisto ne povas paroli la anglan" means that they cannot speak English, but still does not explain why. (Perhaps they are in a place where English is not spoken, so English would not be useful to them.)
"Tio ĉi esperantisto ne kapablas paroli la anglan" specifies that this Esperanto speaker is not capable of speaking English, which finally gets to the root of the cause.
Or, perhaps, "tio ĉi esperantisto ne volas paroli la anglan," and I'm barking up the wrong tree since they don't want to speak English!
In the end, I think this exercise is deliberately left vague to encourage learners to have a conversation about it
I guess I really depends on the situation. You could say Tiu cxi esperantisto ne povas paroli la angla. Which means this Esperanto speaker cannot speak English. Or you could say the other way. Yes you can, for your question. Sorry I am not really too good at Esperanto correct me if I am wrong.