Is it true that Islam was tolerated in the areas reconquered by Christians?
It is not true that Islam was tolerated in the areas reconquered by Christians. Review historian sayings, in the related links, on the behavior of Muslims when got control of Jerusalem and Spain and the counter behavior when Christian regained control for proofs and more details.
The historical record is mixed and depends entirely on a given set of circumstances. While the Christian regimes were typically more Anti-Muslim than Muslim regimes were Anti-Christian, this is not a hard and fast rule.
The During the Reconquest of Spain, period Muslims were permitted to live in much the same way as they had prior to the onset of Reconquest. This period of toleration lasted from roughly 900 C.E. until 1492 C.E. when the Muslims were expelled from Spain along with the Jews.
The conquest of the Ottoman Balkans by the Austro-Hungarians (a Catholic regime which had targeted Protestants in the 1500s for death) resulted in the acquisition of Bosnia and Herzegovina, a primarily Muslim region. There were no attempts by the Austrians to remove or to Christianize this Muslim population.
Of course, as noted above, these are the outliers. The more common Christian action has been to forbid Islam in Christendom up until the post-World War II era.