How to interpret this XML plan fragment?
To answer my own question, my original understanding was correct: the discrepancy between EstimateRows="1" and ActualRows="10190123" was an indication of incorrect optimizer estimates, caused by stale statistics (as suggested by commenters). With updated statistics the node in the plan corresponding to node 6 in the question looks like so:

&lt;RelOp NodeId="25" PhysicalOp="Index Seek" LogicalOp="Index Seek" 
 EstimateRows="1.25676e+007" EstimateIO="23.8363" EstimateCPU="1.78132" AvgRowSize="24" 
 EstimatedTotalSubtreeCost="25.6176" TableCardinality="4.94087e+009" Parallel="1" 
 Partitioned="1" EstimateRebinds="0" EstimateRewinds="0"&gt;
...
&lt;RunTimeInformation&gt;
  &lt;RunTimeCountersPerThread Thread="2" ActualRows="3457399" ActualEndOfScans="1" ActualExecutions="1" /&gt;
  &lt;RunTimeCountersPerThread Thread="8" ActualRows="0" ActualEndOfScans="1" ActualExecutions="1" /&gt;
  &lt;RunTimeCountersPerThread Thread="1" ActualRows="0" ActualEndOfScans="1" ActualExecutions="1" /&gt;
  &lt;RunTimeCountersPerThread Thread="7" ActualRows="0" ActualEndOfScans="1" ActualExecutions="1" /&gt;
  &lt;RunTimeCountersPerThread Thread="5" ActualRows="8053198" ActualEndOfScans="1" ActualExecutions="1" /&gt;
  &lt;RunTimeCountersPerThread Thread="3" ActualRows="0" ActualEndOfScans="1" ActualExecutions="1" /&gt;
  &lt;RunTimeCountersPerThread Thread="6" ActualRows="0" ActualEndOfScans="1" ActualExecutions="1" /&gt;
  &lt;RunTimeCountersPerThread Thread="4" ActualRows="0" ActualEndOfScans="1" ActualExecutions="1" /&gt;
  &lt;RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" /&gt;
&lt;/RunTimeInformation&gt;


Although the problem could be seen in the XML plan representation directly, for those who want a graphical representation there exist a free tool that does not require the SQL Server Management Studio, SQL Sentry Plan Explorer, or even Windows for that matter -- it is an XSL stylesheet created by the StackOverflow user Justin.