You're seeing just the macro refute_sink_buffer, go back to Membrane.Testing.Assertions module for more information.
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refute_sink_buffer(pipeline, sink_name, pattern, timeout \\ 2000)

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Asserts that Membrane.Testing.Sink with name sink_name has not received and will not receive a buffer matching buffer_pattern within the timeout period specified in milliseconds.

Similarly as in the assert_sink_buffer/4 the_sink needs to be part of a Membrane.Testing.Pipeline.

refute_sink_buffer(pipeline, :the_sink, %Buffer{payload: <<0xA1, 0xB2>>})

Such expression will flunk if the_sink received or will receive a buffer with payload <<0xA1, 0xB2>>.