Liberating Insight not a Buddha concept

Because he originally only ordained to “steal” the Buddha’s Dhamma to preach it to people and thus live a comfortable life:

I see. But then, that makes a little difficult for him to be considered to be at that point an arahant, doesn’t it?

Usually wanderers of other sects had to first pass a probationary period of four months before they could be ordained. Perhaps in this case the Buddha saw that Susima was ‘one with little dust in his eyes’ and allowed him to ordain immediately.