There's a massive difference.
On the one hand, Southampton have let most of their best players leave and have effectively forfeited any hope of achieving anything in the immediate future. But they are then singling out one of their most loyal and long serving players and saying to him, "You? No, unlike all the others, you aren't allowed to leave". That's a monumental betrayal in the context of a player's career. Others who have given far less to the cause were allowed their big move to fulfil their ambitions. But the loyal player, who has helped them up through the divisions, is forced to stay and wallow in the mediocrity (or worse) that Southampton will now become. I can't think of any good and ambitious player who wouldn't be totally disillusioned by such a slap in the face.
By contrast, Modric was part of a Spurs squad that was still growing and improving - not being dismantled. There was genuine hope that great things could still be achieved at the club the following season. Indeed, so it proved as Spurs finished fourth and would have finished third or higher had Harry not effectively gone AWOL for the final few months.
As to this story about what Levy supposedly promised Modric the year before, it always sounded weak to me. Players and agents are savvy enough to know that a verbal agreement isn't worth the paper it's written on. They can't have believed that Levy would have acquiesced to any offer from any club. And when that club turned out to be Chelsea.......
