There is nothing special about the toes. If the WR is moving forward towards the sideline and taps his toes and the heels never touch, then it is a catch. However, if the entire foot comes down, all of it has to be inbounds, because it is all one step.
You can’t say his toes touched in before the heel came down out of bounds, it is all one step and it all has to be in. Just like if a guy is going towards the sideline, catches the ball, and then steps halfway out of bounds, you can’t say that his heel was in before his toes touched out. It has to be the entire foot.
It was the right call. Still a great effort by Lamb though.