Whilst I can understand the advice to minimise sparks in the vicinity of possible explosive gases, if you connect the jump leads to anything other than the battery directly you will increase the chances of passing high current through something sensitive. Connecting battery to battery allows your battery to directly supply current to the point where their battery would provide current for starting, without going through any other circuit on your car, or theirs.
Unless you have had a battery on charge for some time in an unventilated area the chances of a build up of significant quantities of explosive gases I would say will be minimal and can easily be countered by opening your boot and their bonnet for 5 mins before attaching the leads.
Good quality leads are a must, heavier gauge the better.
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