Is the development of "developing countries" evidence against the idea that former colonial powers owe their headway to exploitation?
The recent development of many "developing countries" and "emerging markets" does not mean that Western colonial powers did not exploit them in the past.

Emerging economies owe their modern GDP growth to cheap labor, as (former) Western colonial powers switched their (expensive) manufacturing bases to countries like China, India, etc. In the past, they were exploited mainly for their natural resources moreso than labor.

The continued GDP growth of Western colonial powers is due to a shift from manufacturing (labor-intensive, low-education professions) to "high tech" industries which require significantly more education (medical research, computer programming, etc.).

The book "Capital" by Thomas Piketty has a far more detailed history of how capital flows shifted from the 18th century to modern times, and the economic interplay between Western colonial powers and the developing nations. https://en.wikipedia.org/wiki/Capital_in_the_Twenty-First_Century