It means if you have won your past 5 games, you're more likely to have games that tend to be 45/55 in favor of your opponent
No, it doesn't.
There are two ways to make a 50% win rate happen.
What you're suggesting is they are taking players from slightly lower skill ratings and putting them into the games of the player with the winning streak in order to make it harder on them. Multiple players are being moved around in this. We have no evidence of it happening, which could still be done by spreading out the random losses across many lower skilled players, but would take a significant amount of resources to pull off.
What we need is to have players of EQUAL SKILL LEVEL in each team to be the closest possible of FAIR and FUN matches, with a permanent focus on having 50/50 chance of winning for either team.
This is what we have. This is exactly what we have. There is no genius level conspiracy here being pulled off by Riot, there are just people running around not understanding the current matchmaking system.
After a winning streak the matchmaking system views your chance of winning to be 55/45 against the people you have been playing against, so it places you and you alone into a slightly higher tier. Maybe it's wrong and needs to give more time for adjustment, and maybe your win streak was luck, so chances are you're going to drop back down to where you were by losing. There is no way to build a matchmaking system that attempts to make matches fair that doesn't end up doing exactly what the current matchmaking system does, at least so long as it is attached to the current ranked ladder.