Well, Bayern is always the great provider for the German national team, so it gets much easier when the national team and the main provider play in the same style. I think it's fair to say that Germany couldn't have played this well in this style without Bayern doing well in this style, so both Guardiola and Heynkes had an impact on the style and the execution of Germany. Not as big as an impact as LvG though, because he was the one who had the biggest impact on Bayern's style of play by changing it radically. The German association had already decided for an attack minded style in 2000, with new plans for youth development and competitions, but it would have been more difficult to be succesfull with it if the biggest provider of players had continued to play defensive counter attacking football.
Spain is a different story, of course as a vigorous proponent of youth development he has had a positive influence on La Masia, and La Masia was very import for Spain's run of successes. But Barcelona was already busy with youth development and the playing style goes back to Cruijff and even Michels, so his influence was more limited than it was in Germany, and not as big as Guardiola's. Because Guardiola was the manager in charge and the one having the players playing together like that at Barca. And allthough Del Bosque could use all the training work of Guardiola, he still had to deal with the old Spanish problem of having players from different clubs and regions playing togehter as one team.
But it's nonsense to say that a club manager can't have an impact on a national team. The Dutch national team that did well in 98 and 2000 had Ajax written all over it, it happened to be LvG's Ajax. Euro 96 was even wrecked by internal Ajax troubles about difference in wages and leadership.
Off course when club managers just stumble upon a style of play instead of working on a specific one, and players play purely on their instinct, must'nt be "stifled" with tactical instructions and should be free to do what they want without thinking, a club manager can't have an impact on the national team. First the manager has to implement a specific style. But a national team like that won't have any impact on a World Cup or European Cup anyway, so all those club managers can only be a small part of the failure of England.