Nico Reillu (2) and Kevin De Bruyne were City's goalscorers in Saturday's fifth round victory.
Guardiola said afterwards: “I didn’t take anything for granted, when I saw how well they defended and the problems they created for Liverpool. They’re an incredibly solid team.
“We created a lot and conceded little – the basics we have done for many years. We’re happy to reach the quarter-finals and with the way we played.
“How many times in the last years have we reached the semi-finals? When you arrive six times in a row in the semi-finals, it’s because it is important. When you play in this competition and you behave like the way we behave, I’m so proud – I love it.
“It shows how honest we are, and I like that. We’re one game away from Wembley and we’ll see what happens.”
On O’Reilly, he said: “He’s not a typical academy player. He’s a physical presence – a number 10, I would say. He can play in a few positions. He’s a threat from set pieces because he’s strong. I’m really pleased for him because he helped us a lot.”
On Plymouth, Guardiola added: “Defensively they were incredibly well organised. They didn’t press much and defended a little deeper, but they defended with solidarity. That’s why I said I didn’t take anything for granted.”