On the presentation Dominik describes the transaction initiation process, in which the second step is random tip selection, where a tip according to the definition is an unconfirmed transaction. On the diagram the tips are colored in grey, and they don't have incoming arrows. If every participant in the network uses the same MCMC algorithm to choose two random tips out of all sets of tips, then no node should have two incoming arrows.
On the diagram all green and almost all red of nodes have multiple incoming arrows, which means the MCMC choose nodes that have already been confirmed by at least one node. If I am reading the picture correctly, the tip selection algorithms does not select only tips, but transactions that have not been fully confirmed. That is a new node runs a MCMC algorithm to choose two previous transactions to confirm out of the set of grey and red transactions.