How does someone get to platinum so quickly???

It is typically just a matter of where a player is skill-wise.

I played the game for about 2.5 seasons before I ever bothered playing ranked games. When I originally started last season, it was mainly a way for me to practice new champions without getting crushed.

At that point, I was getting matched primarily with Diamond and some Master players in Normal games, so it was easier for me to practice new champions against Silver and Gold players in Ranked.

After 300 games, I'd probably accept that your current ranking is fairly accurate and start looking for areas in which you can improve.

Taking a quick look at your match hisotry, I would probably recommend you try to improve your CSing and item builds. Many of your builds are rather questionable, such as:

with Stormraider's Surge

with Bond of Stone (Against a team which deals a lot of magic and true damage)

I'd recommend looking at champion.gg and probuilds.net for item builds until you figure out why certain items are build on certain champions in certain situations.

I'd also recommend using those websites for rune and mastery pages.