Postgres heap vs SQL Server clustered index
PostgreSQL simply doesn't implement this feature.  There is no trick to not implementing it.  It isn't implemented in the straight forward, uncomplicated way of just not doing it.  To use one bit of jargon, all btree indexes in PostgreSQL are "secondary indexes", not "primary indexes".  Even the primary key's index is a "secondary index".

There are some cases where clustered keys (or index organized tables, as another product calls them) are important, and in those cases PostgreSQL fails to "supply exceptional performance".  You can argue about how common those cases are, of course, but they certainly do exist and it is unfortunate that PostgreSQL doesn't offer a solution for them.  There have proposals to address this, but I don't think any of those efforts are currently active. 

In some cases, you can ameliorate the problem by using the CLUSTER command, or by implementing partitioning, or by using covering indexes, but none of these is entirely satisfactory as an alternative to real clustering.