The thing about having his back is the thing that has always been true of Clark having his back, namely that he will also call him on his shit and do what he needs to even if Bruce wants to fricking murder him. That offer isn't just for kisses and cuddles and holding his hand. It's also 'I will talk you off a ledge' and 'I will smack you off a pedestal' because that's what having someone's back means.

The kiss doesn't take him by surprise, and he accepts it in the spirit in which he feels it was given. They're both here and they're both alive and they're together. I've got you and you've got me and we're stronger together.