I am a convert. When I converted, my confessor, who was a Monsignor priest of our parish did our confessions in his office. He stated as a convert, all of my sins need not be verbalized. He asked me to bring the sins I was most concerned about to the front of my mind. He layed hands on my head and offered a prayer (this was at least 15 years ago so I don’t remember the exact words). His prayer was basically calling on God to absolve the sins I held in the silence of my heart. He explained it was called a blanket absolution. I felt the relief if forgiveness. I have always had the question if that absolution was valid. I was given a penance and he said as I did my penance to meditate on the sins he absolved.
My presumption is that this is just a case of overly simplified wording.
The only sins we are absolutely required to confess are mortal sins. Mortal sins are actions that are gravely wrong that we knowingly and willingly chose to do. Obviously such sins would be the ones you would be “most concerned about” and that would be in “the front of [your] mind.”
For an adult convert it can be a bit overwhelming trying to think of every sin they ever committed in their 20, 30, 40, 50 years before conversion. The priest was just trying to simplify the process for you so that you wouldn’t be overwhelmed and confused.
While changing the words prior to the words of absolution is illicit, as long as he said the words “I absolve you” then it was perfectly valid.