Christianity is a monotheistic religion.They are not Christian. Few if any of those countries actually live up to the label of monotheist.
13 of the 20 countries listed identify Christianity as the largest religion. 5 identify Islam as the largest religion. The other 2 have significant populations of monotheist religions.
For example #1 on the list Luxembourg;
Church in Clervaux, Luxembourg
As of 2018, 73.2% of Luxembourg's population adhere to forms of Christianity (63.8% are Catholics, 1.8% are Protestants, 3.0% are Orthodox Christians while 4.6% adhere to other Christian denominations). 2.6% of the population are Muslims and 0.6% are followers of other non-Christian religions. 23.4% of the population do not have a religion.[2][3][4]
Religion in Luxembourg - Wikipedia
en.m.wikipedia.org
I could go on but I don't think you are interested in examining the facts.