He didn’t, actually, compare Starmer to the IRA.
While it’s nonsensical to infer, as Johnson seemed to, that Starmer supported the IRA, I think what he was really getting at, rather clumsily, was that Starmer (and others) served in Corbyn’s cabinet. However apart from the fact that its a rather desperate attack, which as I say rebounded on him, it is true that some Labour people decided that it was best to grin and bear their leader while they could. Even to the extent of serving in cabinet under him. The alternative tho would have been an even worse opposition and possible a split in the party.