Judging by the last 5 games you've lost they did make mistakes but either you made more or didn't spot their mistakes.
Mis-rating of players.

Maybe I just had a miraculous growth spurt. But some of those 300s are surprisingly sophisticated (especially in Rapid.)

Most people on chess.com think 300s play like chimpanzees.
But a 300 on chess.com would beat 99% of "I play when my grandpa's in town"-type players.

Alot of new members may start off lower and the rating evens out over time,
https://support.chess.com/article/208-what-is-a-sandbagger
If maybe you think people are sandbagging a rating please report them.

Here I lost to many 200s who played like 1000s . Either the rating system of here is flawed or these 150-300 rated players are just brilliant. They do not make mistakes or blunders . Strange.
How do you know what a 1000 plays like if you are 200? Stop complaining. If you suspect someone of cheating, report. Simple as that.
Here I lost to many 200s who played like 1000s . Either the rating system of here is flawed or these 150-300 rated players are just brilliant. They do not make mistakes or blunders . Strange.
How do you know what a 1000 plays like if you are 200? Stop complaining. If you suspect someone of cheating, report. Simple as that.
But it's wrong on this site
Here I lost to many 200s who played like 1000s . Either the rating system of here is flawed or these 150-300 rated players are just brilliant. They do not make mistakes or blunders . Strange.