Note: as @Acccumulation pointed out, the video mentioned in this post is using a bad example, plz consider the example in the answer.
This video (https://youtu.be/gvG713JZPL4?t=264) says
only can be a conjunction
and gives this example
you just started learning English only you sound fluent because you're so confident
Although I did see "only" connects 2 parts of the sentence, I cannot understand the meaning of that. I cannot even understand the meaning of whole sentence.
Pretend that I started learning English a few days ago and my teacher said that to me. I guess she is trying to encouraging me though, I still get the meaning clearly.
I guess "only" here means, but or however, with which
you just started learning English but you sound fluent ...
makes more sense. and I cannot yet get the point.