Yep, it was a dive, and this "After Further Review" basically showed the NFL's cards...

If there was any doubt left, this play took care of it.  Again, can't embed -- link to YouTube...

First drive of the game, 4th and 3 just inside the San Fran 40, and that is ruled and kept a catch by DeVonta Smith.

Smith knows it's not a catch -- and if you watch closely (probably at about 48-50 seconds is the best look you'll get -- watch where the ball is "in his arms"), you can see why.   So not only a magnet "catch", but also a non-catch that Smith knew wasn't, so he bolted the team to the line to get the next play off before SF could challenge it.