The reason why your comparison is not being responded to well is that +/- is easily affected by context, and only shows part of the picture.
1) Seider's most regular defensive partner for much of the season, particularly early on was Lindstrom - a rookie in a new country who struggled at times until he found his feet. Bjornfot's partner for most of the year was Ladue, a 27 year old who's pro career was an even split between NHL & AHL, after a full college career and the best D man in Ontario.
2) Seider's WJC showing was a massive plus, as it gave us a chance to compare him with his peers globally and he was as good as any other D in the torunament
3) From what I can find out it seems Seider got more defensive assignments playing with weaker lines, particularly early.
This is not to dismiss your wider opinion. Even looking at the stats, Seider was and remain a slight reach at #6 - although looking at others in the draft doesn't look too bad (perhaps Cozens, Knight, Newhook, Zegras & McMichael could have been better picks, but none have played pro yet, and as wings fans we all know what its like to have kids killing it in junior and then flatten out badly at the AHL level). Also Bjrornfot was a player I liked and should have gone higher. Had him ahead of Thomson, Harley and York, and had we been picking 15 ish, it would have between him and Newhook for me. So he could indeed prove to be a steal if his development is at least linear going forwards.
The comparisons you show with AHL D+1 years actually show why people are excited about Seider, and indeed why kings fans should be about Bjornfot. Liljegren was a hyped offensive D with worse point totals/ratio whose +/- is massively skewed by the strength of the Marlies D that year - I think his partner was +30 something, as were about 4 d-men in that team. Risto has problems in his own end, but has talent through the roof when one takes his physical potential into account, and perhaps if developed in a less badly run team could be a top pairing guy. He still might get there. Lindholm (same pick) was made as an immediate comparison to Seider, and the early signs are Seider has a bit more puck skill and is more physical but not quite as positionally strong. I think we'd be happy if we ended up with a more dynamic Lindholm.