That religious freedom was enshrined in the Constitution, does not mean that the nation was not a Christian nation.
Since no one had to be a Christian to be an American citizen under the new Constitution m, there is no way it truthfully can be said that America was founded as a Christian nation.
Sure it can. The nation was massively majority Christian, with Christians dominating every aspect of public life and with Churches happy to engage in politics and Christian ideas and culture enshrined in law and culture.
That it also respected the religious freedom of every citizen, including religious minorities, does not conflict with that.