Learn English – meaning of the phrase “telling effect”

It is telling to note that ... is often used to draw more attention to a point, often with negative connotations.

In your example it would be factual to say

"The language of exclusive territoriality in disputes such as this one originates with local authorities."

But the addition of It is telling to note that ... implies that the local authorities may not be acting in line with a higher authority.

A different example might be

The doctor did not perform a full examination prior to the patients demise.

This simply states that the doctor did not perform a full examination. However, adding in the phrase ...

It is telling to note that the doctor did not perform a full examination prior to the patients demise.

implies that the doctor should have performed a full examination and had he done so the outcome may have been different.

"Pushing" in the context of giving opinions or views carries a context of trying to persuade others in a forceful way. A person may be described as "pushy" if they are excessively assertive about any subject.

Someone described as "pushing their opinions upon others" means they are trying to persuade others to take on the same opinions, or perhaps trying to present their opinion as the only valid one.

eg.

"Please stop pushing your views about veganism on me."

It is somewhat different to the term "peddling", an old-fashioned word for "selling" that has come to be a pejorative term describing someone who spreads their views or ideas:

eg.

"A former Alaska lawmaker has pleaded guilty to a state count of letting unregistered lobbyists peddle their ideas to him."

"Peddling" carries the idea of spreading views to as wide an audience as possible, whereas "pushing upon" more suggests trying to persuade a specific, possibly captive audience.