Of course, we should understand that there are false notions of ecumenism but authentic ecumenism was prayed for by Jesus Himself before he ascended to heaven. It is INDEED important and willed by the heart of Jesus Himself.
How is the ecumenism referred to by Pope Francis, per Mr. Mallet, any different than that which Jesus calls for?
Here you can find more information/analysis by Fr. Z (Mortalium Animos) which is good:
http://wdtprs.com/blog/2011/02/podcazt-166-toward-a-true-ecumenism
Because Traditional Catholic Teaching states that the Church is already united in the Catholic Church. This is not what the post-conciliar popes/Vatican II teaches. Post-conciliar popes also take part in non-Catholic worship which is forbidden pre-Vatican II.
These teachings didn’t all of a sudden change in the 1960’s.