Last season I lost 6 of my placements and still gained 150SR (which was the most I'd seen anyone claim to gain on here or the official boards). I really don't believe W/L has any impact on your placement at all. It does seem absurd that 10 matches would in any way 'overwrite' significantly the performance of potentially hundreds of matches in a previous season. What had actually happened was I'd lost a lot of SR in a loss streak prior to the season's end and to some extent the placements were able to fix that (though, by contrast had I won 8 matches in placement like I did in S3, I'd have gained much more than 150SR in normal matches, when in fact I lost a small amount)...
When Kaplan was talking about W/L in that way, I don't think he was referring to placements.