Brentford actually played a defensive game in the second half, just like you said. Chelsea did exactly what you said in the second half. Chelsea scored twice in the second half to take the lead against Brentford, but Brentford levelled up against Chelsea in additional time to deny Chelsea all three points. Well before Brentford vs Chelsea started, I already made it clear in this thread that Chelsea wouldn't defeat Brentford, and the game played as I expected.
Chelsea did everything they could but in the end luck was not on their side. At first I thought Chelsea would play with confidence because they won their previous 2 matches but Brentford know how to handle Chelsea, this draw brings Chelsea down to 4th place and this is also an indication that they are not fully ready to compete or maybe we should wait for them to fully adapt. Chelsea next challenge will be playing away against Manchester United, I can't predict anything because I think both teams are in almost the same situation.