Interesting one isn't it
A few other refs have tweeted it was a penalty due to the foul continuing as the player went into the box, stating Stevens still had contact with Solanke when he slid in.
Colin Murray on Quest said the rules say any foul that carries on into the box is a penalty is a foul also and was very dismissive of any saying it was not a penalty. He says it was correct based on the rules but above suggests possibly otherwise.
I have to say we had a Sharp one the same the other week and a free kick was given not a penalty.
It does seem daft to have such a rule for holding but not for other tackles where the foul carries on. Probably more obviously than today where I am not even certain the contact carried on much after he got in the box but just seems odd to differentiate. You could slide in and foul a player,then carry on bringing down but get away with it if the player finally gets brought down in the box? Yet a grapple or hold treated differently?