The claim begins with the song The Grand Conjuration off of The Ghost Reveries record. This was a divisive song for Opeth fans.
The Grand Conjuration came out on July 26th, 2005. It was released as a single. The teaser track to the album Ghost Reveries. This was their first album with a major record label, Roadrunner records. This song and this record represented a major change in the band and it's members lives.
Peter Lindgren leaves Opeth in May 2007. He was quoted as saying "The reason behind this is that I feel that I simply have lost some of the enthusiasm and inspiration needed to participate in a band that has grown from a few guys playing the music we love to a worldwide industry. The love of music has always been, and still is, our motive, but I personally have lost something along the way. The massive amount of touring has taken its toll."
Watershed is released June 3rd, 2008. On it, the track "Porcelain Heart." A selection of lyrics from Porcelain Heart:
I lost all I had (That April day) / I turned to my friends (Nothing to say) / I wallowed in shame
I posit that Peter broke the news to Mikael in April, before departing officially in May.
I said that I love (Eternal schemes) / I clinged to my past / (Like childish dreams)
Icy roads beneath my feet / Led me through wastelands of deceit / ...Kept inside our idle race / Ghosts of an idol's false embrace
Peter's reasoning for leaving the band seems echoed in these line. Mikael wanted the rock-star dream. Peter’s statement reads like the cost of that dream: the band had become a “worldwide industry,” the touring machine had taken its toll, and the thing that began as friends playing music had become something Peter no longer recognized himself inside.
So why do I mention, specifically, The Grand Conjuration?
Well, Porcelain Heart seems to be its reprise. The main driving chord theme of the chorus in Grand Conjuration makes a very slightly altered appearance on their next (Peterless) record. As a Bridge/Outro.
The Grand Conjuration Main Chorus Chords Just listen til the repeat.
I posit this music coincidence is not an accident. Mikael was as cryptically and as clearly as he could muster, saying "I am so sorry." to his best friend whom he drove away chasing his rockstar dreams.
Akerfeldt himself said that the contents of Watershed were very personal, and had to do with family and lineupchanges:
Akerfeldt on Watershed: "This one was all personal lyrics." He said it wasn't a concept record - the songs came out of what had happened in his life, namely becoming a father again and the lineup changes (i.e. Lopez and Peter leaving) source
For those that don't know, since leaving Opeth, Peter Lindgren has become the Vice President of King. They made Candy Crush. He's doing just great.
So let me unpack it a bit. The band almost broke up recording Deliverance/Damnation, but iirc, it was a mix of Martin Lopez and his overwhelming anxiety, struggling to finish recording tracks, and that Peter had seemingly checked out.
Then during Deliverance, Akerfeldt said:
“Peter pissed off right as we were about to start recording the guitars: ‘I’m going to a party, I’ll be away for a week or two.’ I guess he lost interest. He didn’t really play rhythm guitar on the records. I did most of it because he wasn’t there.” source
"[Peter was] showing enormous disrespect to me, to the band, to himself, as a guitar player" source
But that was all before the records/songs in question. Long after Ghost Reveries, in 2021, Akerfeldt said (beginning to soften his harsher takes on Lindgren, and own some fault):
"On Still Life, Peter didn’t contribute at all. He would never say anything, but I could tell he was not happy about that. I think it made him feel small, like I didn’t allow him. On Blackwater Park, he wrote a couple of riffs that we put in the heavier verses of Dirge For November, and I wrote the rest.” source
Ultimately, Mikael expressed regret over the way he had treated Peter.
Perhaps most strongly to my point (going to edit my original post with this probably):
Akerfeldt on Watershed: "This one was all personal lyrics." He said it wasn't a concept record - the songs came out of what had happened in his life, namely becoming a father again and the lineup changes (i.e. Lopez and Peter leaving) source
Funny enough, this interview that touches on the Mikael and Peter relationship seems to feature Porcelain Heart. Maybe someone else shares my theory.