You need to view the combination of verbs in the same sense that we understand futur proche (Near Future). Il va parler = he is going to speak. Il allait parler = he was going to speak. The emphasis is not on the "going" but that--together with the infinitive--is indicates the impending action of the infinitive.
There's a distinction in English between "he was to speak" (ie: he was scheduled to speak, but usually the implication is that he couldn't or didn't, he became sick, the conference was cancelled, etc) and "he went to speak" (he went somewhere for the purpose of speaking), but I'm not sure how that difference would be captured in French (and I'm not convinced the Duolingo answer is correct).
When you say "he went to ...", it suggests that he actually went to (some location, i.e., not here) in order to do something. If that is the true meaning you want to convey, you would say "il y allait pour parler". If you want to say that he was scheduled to speak or was expected to speak, you would have to reframe that in French. The expression "il allait parler" is like near future but it is referring to a near future in the past, i.e., he was going to speak.
That's not the only way to interpret "he went to speak" tho. It can also mean that he was about to speak and also implies that there may have even been a visible sign that he was about to speak like opening his mouth.
I still agree tho, even with this interpretation, there is a distinction between "he was to speak" and "he went to speak". "He went to speak" would be far more immediate whereas "he was to speak" could mean that he was scheduled to speak at some point (possibly far) in the future
I'm assuming you mean if you were given the audio. The answer is - almost. If it was plural there would be a Z sound between "ils" and "allaient". Il-Z-allaient. The "Z" sound that joins "il" and "allaient" (so that when said in regular speech it almost sounds like it's one word) is the only difference. When pronounced by themselves, "ils" and "allaient" sound identical to "il" and "allait".
Does that help?
Past imperfect (l'imparfait) form of "aller" + infinitive form (l'infinitif) of "parler" = "was to speak". "Was to speak" in English can also be said as "was going to speak," depending on context. Verb tenses do not always translate perfectly between the two languages.
For instance, "je mange une pomme" can be translated as "I eat an apple," or "I'm eating an apple." (There are more exact ways to say some things, like "Je suis en train de mange une pomme" more specifically/exactly expresses "I am eating an apple (right now).")
I think you can think of it as "future in the past."
He was on the point of speaking = he was going to speak. This is from our current point of view, in the present.
When the incident occurred (that he should have spoken, but didn't, despite being scheduled to speak), the speaking was indeed 'future' :
He is going to speak / He is scheduled to speak (so "speaking" at that point is future).
As far as verbal communication goes: Talk = Speak.
In both cases, audible words are coming out of your mouth.
Many people on this thread are analyzing whether they've heard "He was going to talk" before; or, if it's formal or informal; or if it correct or incorrect.
It may be unheard of or sound weird to native speakers on both sides of the Pond but "He was going to talk" is taught in English Language classes all over the Globe as correct Standard English. And it's not just a thought, it really is.