50% winrate doesn't imply WLWLWL, there will be some win streaks and loss streaks so you'd eventually pass promos. Heck, if you maintained a 67% winrate in regular games and a 10% winrate in promos, you'd still eventually pass promos.
And even if it did imply a WLWLWL... string, your mmr would steadily climb up in the 67% winrate part of the regular games, until eventually you just skipped the promo series, which is possible if your mmr is sufficiently high.
By contrast, if you maintain a 50% winrate in regular matches following an unrealistic WLWLWL string and 67% in promos, you'd climb for a little bit, but eventually you'd get stuck never actually hitting promo series since you'd get to the point where your MMR and rank 'match' so LP gains=LP losses.