Truth being, none of us have all the knowledge and all of us can be in error about something without making us bad people or even stupid people.
That is, Fr Amorth is likely a good priest but like the rest of us, has his biases. Chances are he's never done yoga and/or he may have observed some folks who were possessed or obsessed who happened to do yoga and he made a connection (for example had he asked, the possessed or obsessed person might have ridden a bicycle as a kid also, so he could have said "riding a bicycle leads to satanic influence").
Truly those type of errors are common and even made by our scientists as shown in the epidemiological studies which "proved" that giving birth control meds to menopausal women protected them from heart attack -- until 2 clinical studies found the opposite i.e. that birth control meds greatly raise the risk of not only heart attack, stroke and thrombosis but also significantly raise the risk of breast cancer. (One magazine pointed out two studies, one in the 1930's which gave synthetic estrogen to rats and all of them, male and female came down with breast cancer and yet, large organizations, today, ignore the connection completely).
As an aside, our wonderful news services, which are so quick to condemn the Catholic church have failed to report the dangers of even low dose birth control to the public with the result that last I checked, there were millions of women still taking birth control medication, both for family planning and for hormone replacement therapy!
Sticking with the Bible and the words of Jesus, is a good idea in the case of Yoga ! I am a devout Catholic who DOES yoga several times a week. I am also 67 years old and fit and yoga is part of my program. Yoga is a wonderful inheritance from the East and can be a healthy practice. We do not have to embrace other religious beliefs to learn their good health practices. And bottom line - the words of Jesus "other sheep have I who are not members of this flock" and "Nothing that enters one from outside can defile that person; but the things that come out from within are what defile." Mark 7:15
Postscript: Fr Amorth also said that the Harry Potter books can lead to evil. I tend to feel that, at best, those books are a horrible waste of time for kids who should be reading real literature like Tom Sawyer and Booth Tarkington. I read a couple of Harry Potter books and while having a total absence of God, they encourage magic, some of it, black magic which could only have originated in Satan and which really can likely not lead to anything positive. Luckily, most kids find these books boring - at least that was true of my grandkids. Perhaps the answer to Harry Potter is for parents and teachers to gently crowd them out by suggesting worthwhile literature for children. My emotional reaction to reading Harry Potter books was that they gave me a kind of creepy feeling.