Like i said, his main focus is SP. SMP isnt even meant to be out, he wants to get past alpha asap so ofc hes gonna get as much into SP as he can
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He should be doing what other devs do, create it exclusivily for SMP, but then for SSP just create a local server and connect to it, it saves a lot of time and effort (you effectively make two games... whilst only making one). A lot of the top Valve games do this for instance.