He actually abandoned his principles for that Real defeat though.
He ended up going with this 442 variant and Bayern looked terrible of course.
Guardiola has since referred to that second-leg defeat as “the biggest f***-up of my life as a coach,”
also saying: “I got it wrong, man. I got it totally wrong. It’s a monumental f***-up. A total mess. I spend the whole season refusing to use a 4-2-4. The whole season. And I decide to do it tonight, the most important night of the year. A complete f***-up.
“
"There’s no doubt that Guardiola did indeed get his tactical set-up wrong, personal tragedy between both legs perhaps clouded his mind although he’d never admit it. But what is also true that his players, who lobbied Guardiola hard to play that same 4-2-3-1 (or 4-2-4 as Pep calls it) all season utterly failed to deliver when given their preferred formation."
https://www.squawka.com/en/pep-guardiola-bayern-munich-real-madrid-disaster-champions-league/
I wouldn't pick that game to criticize his methods. Pick the game at the Nou Camp.
Bayern are decimated with injuries. Pep goes there and man marks MSN at their pomp for most of the 1st half.