The guides I have read still recommend attaching the negative lead to a good earth on the car bodywork/engine rather than the negative battery terminal - or, as in your photo, somewhere that is virtually the same thing. .
I am not an expert but I think there are two reasons for this. Firstly it bypasses the possibility that the negative battery lead itself is making a poor connection to the bodywork/engine. But mostly because it minimises the chance that any spark produced when connecting the lead could ignite explosive gases produced by the battery.. This may be why they often recommend connecting to a point some distance from the battery. But the short leads on these devices dont give much option beyond where you have indicated.
Although a modern sealed battery is unlikely to produce inflammable gas in explosive concentrations they cant be sure the battery being boosted isnt an older type. I think the guides err on the side of caution. And therefore so will I
