"can confidently assert" is grammatically correct and can be used in written English.
You can use it to express that you are able to confidently state or declare something. Example: "After conducting extensive research, I can confidently assert that climate change is a very real and pressing issue."
In the same spirit, we can confidently assert that La Grenouillère is one of the few genuinely first-rate avant-garde, modernist restaurants in England, with the tiny proviso that a pedant might technically regard it as being in France.
So to the question 'Who owns culture?' we can confidently assert the truth of one response: not the nation state.
As a result, no one can confidently assert that colleges today are helping students to write better, speak more eloquently, think more rigorously, or reason quantitatively more proficiently than they did in the 1950s.
Therefore, in view of the phylogenetic evidence and positional information, we can confidently assert that the miR482/2118 gene family diversified prior to speciation within Solanum.
In the aftermath of what has happened recently, to countries and financial institutions alike, can investors still confidently assert that doing business in Africa carries a comparatively unacceptable level of risk?
An assessment of the interval within which the values of the error can be confidently asserted to lie in is required.
It can confidently be asserted that certain types of consonant clusters, such as tr, pl, dr, and bl, were unknown about two millennia ago.
And that is the reason why we can look at our hands and think about our brains and confidently assert that they are, indeed, the result of natural processes rather than the direct design product of a supernatural God.
None of them could confidently assert that, or describe how the food tasted better.
It is hard to find anyone who will confidently assert Maharaj's guilt.
Those who confidently assert that centre-left voters will desert Labour in favour of the Lib Dems are woefully misguided.
