It means he can get into the front quicker, capable of using charge to pin someone, take them away, and have the option to either slam into a wall/toss them over into environmental without committing suicide/or stopping short before earth shattering.
Two charges of fire strikes also means he can constantly poke at ranges, build earth shatter, and finish targets that are just away from his hammer.
The idea isn’t for Rein to hold his shield, they’re making his attack options more better so he can go in and mess the enemy up faster. That’s also not getting into the passive of his that makes him resistant to CC (And stacks with the role passive.), or they’re giving him a new passive.
It’s no mega buff, it’s changing the perception of tanks being meat shield and into fat aggressive DPS which does tanking as a sidejob.
Does it means he’s not a tank? No. Is it absolutely a step away from what the role intended, absolutely. They’re making Rein fun to be aggressive with so more DPS players pick him up to be that silly charging suicidal Rein because we’re severely lacking tanks and DPS players are more likely to pick that instead of Rein being shield man.
That being said, tank players will still be valued as knowing when to shield, be aggressive, and knowing when to charge/ult is going to make tank players feel more impactful.