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That's not entirely true as that would mean a TTL output without anything connected would just sit at whatever voltage it was sitting at before (like near 0V if it was pulled low before).When the TTL output is high, its output transistors are not trying to pull current into the output pin from the positive side of the power supply. As far as the pull-up resistor is concerned, its lower terminal is connected to an approximately open circuit.
That's why I qualified the statement with "As far as the pull-up resistor is concerned" which was meant to indicate that the output does not necessarily behave like an open circuit in all respects. And we were talking only about the high state then.That's not entirely true as that would mean a TTL output without anything connected would just sit at whatever voltage it was sitting at before (like near 0V if it was pulled low before).