He also committed a chunk of the initial funding. And it's not rare for capital providers who are there on day #1 to call themselves and be referred to as co-founders.

Essentially it is a short-hand for 'those that were present on the founding day of the company and whose names appear on the founding documents besides the lawyers'.

If you get added later on then you are technically not a co-founder, though even there sometimes (and sometimes rightly) exceptions are made.