The phrase "agree with that assertion" is correct and usable in written English.
You can use it when expressing your concurrence with a statement or claim made by someone else.
Example: "After considering the evidence presented, I must say that I agree with that assertion regarding the impact of climate change."
Alternatives: "concur with that statement" or "support that claim."
Few dispassionate experts would agree with that assertion.
In his postgame comments, Ryan seemed far less distressed than he was after last Sunday's debacle in Oakland, though players did not necessarily agree with that assertion.
Indian officials agree with that assertion, saying that despite a major decline in infiltrations by insurgents compared with last year, there has not been a corresponding drop in violence.
I would agree with that assertion.
I don't agree with that assertion.
But according to the data the world doesn't agree with that assertion at all.
Some protesters agreed with that assertion.
At another hearing, a number of senators agreed with that assertion, questioning the whole notion that the nation's underpaid and overworked nurses and police officers should be performing the front-line duties meant for the military.
Some also fear that it gives husbands a chance to betray and agree with the assertion that it creates sedition in society.
These studies agree with the assertion that when the Zn1 xMgxO conduction band is raised sufficiently such that only a small density of tail states are able to accept electrons from the PbSe, charge generation in the PbSe kinetically outcompetes charge transfer to the Zn1 xMgxO such that space-charge accumulates in the device under illumination and increases the amount of recombination.
It is true that the inconsistency between codon and anticodon modes of recognition remains to be resolved, but I do not agree with the assertion that "objectively, we should accept FAM as the most likely model for the emergence and evolution of translation".
