SRE is a discipline that focus on the whole lifecycle of the product. It's not just dumping software and moving on.
Engineers in SRE teams can work with those that develop product, for example, adding infrastructure pieces.
Reliability is "The probability that [a system] will perform a required function without failure under stated conditions for a stated period of time,"
Focus of SRE: improve design and operation to make systems more scalable, reliable and efficient, but this effort is not infinite. Once it's reliable enough, we move on.
Even for small companies, the earlier you care about reliability, the better.