From the reference manual section 5.9:
"The operators <, >, ==, >=, <=, and != compare the
values of two objects. The objects need not have the
same type. If both are numbers, they are converted to a
common type. Otherwise, objects of different types
always compare unequal, and are ordered consistently
but arbitrarily."

The last sentence may apply to built-in types, but '<'
may be evaluated via __lt__ for user defined types, and
there are no such restrictions as I read the
documentation.  If this section is only referring to
built-in types it shuld be clearly stated.

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Thanks for the report, fixed in rev. 46949, 46950 (2.4).