They understand the mobile experience better than most brokerages, and they understand that mobile is where most users are spending their time.
However, IB is decidedly not meant for the young, millennial, small amount of investment principal, getting started with investing/trading segment that Robinhood has carved out for itself.
So Robinhood's angle isn't just mobile first, it's mobile first for the segment of the population that expects such a product experience, who are going to grow their portfolios and trading activity in the coming decades.