I'm reading that Starmer is saying (or will say) that if fined he will resign. In terms of party politics, just neutrally seeing it as "the great game" or some such, it seems like a good move doesn't it?
Two possibilities really
a) He doesn't get fined - which seems fairly likely as, from the scant attention I've paid, I understand the rule was not in place at the time - but his willingness to do the right thing will increase the pressure on his opposite number who has been fined and may be fined again.
b) He does get fined and resigns which you would think ought to make Johnson's position - as someone who has done worse, many times - untenable.
Basically for those who like party politics and enjoy watching this kind of stuff as if it were I Claudius or Game of Thrones then it looks like a brilliant move - one that has taken a concerted attack from the Tories and their allies in the press on his honesty and ethics, along with political pressure on the police, and used it to actually strengthen his claim to these qualities while further exposing Johnson's complete lack of same.
For those who feel that an MP ought to be someone who believes in something and fights to make it happen then perhaps it's not quite so important. In fact I wouldn't be surprised if, amongst his enemies on the left, it simply augmented the idea of him as a bloodless, calculating career politician who has no particular belief in what Labour is supposed to stand for...