I can see why a lot of folks think this... and I know that retconning isn't always favorable, but:
* in "Angels of Darkness," author Gav Thorpe had a Fallen named Astelan gets taken to The Rock and basically suggests that Lion El'Johnson paused to see who was going to win the Heresy... which has stuck with a lot of folks since that book came out
* however, in the book "Unforgiven," it's shown that Astelan was an almost pathological liar and nothing he said supports the First Legion waiting to see who would win... and in all of the Horus Heresy novels (again, I get why folks don't like the retconning) the Lion is anything but waiting to see. They, like other Loyalist Chapters, were purposefully spread hither and yon so the Traitor Chapters would have an easier march on Terra: Ultramar separated by the Ruinstorm; the Blood Angels getting sent waaaaaaayyy out for an ambush; the Dark Angels getting sent wayyyy out and having the Night Lords and Death Guard harassing/following/attacking them, etc.
* proud? Oh yes. Wanting to be the new Warmaster since Horus turned? Yep, that's why he tried to get Perturabo's vote in exchange for the siege engines... whoops, he didn't know Big P had turned. Don't forget also that Kairos tried to turn the Lion and failed, finding him unwavering loyal to the Emperor. Luther and his cohorts rebelling and turning was a HUGE issue, because the First Legion knew what was going to happen to them all if that secret came out. Thus, secrets upon secrets upon secrets. The cover-up is way worse than the crime!
But again, I totally get that all of this is with the new Horus Heresy books and thus has changed a lot of the "old story." But the Lion is painted as nothing but a Loyalist in all those books. Further details if folks want... but instead of "waiting and see" don't forget that in all the new books it's way different:
*** SPOILER ALERTS FOR THOSE NOT READING UP WITH ALL THE NEW HORUS HERESY BOOKS/TIMELINES ***
Following the Trial of Curze, Sanguinius, Lion El'Jonson, and Guilliman all agreed to try and breach the Ruinstorm to reach Terra and aid the Emperor who they now knew still lived. In the Ruinstorm, the loyalist fleet came across a variety of horrors and word of an entity spreading destruction known as the "Pilgrim". Not even the Tchulcha could navigate the Ruinstorm, frustrating the Lion. During the Battle of Pyrrhan the Lion commanded Dark Angels personnel as Sanguinius received a vision and he realized that he needed to go where this struggle had begun, Davin. Reluctantly, Guilliman and The Lion agreed to trust in Sanguinius but both had thought they would simply destroy the world upon arriving. While over Davin, Sanguinius shocked The Lion by boarding the "Invincible Reason" and taking the captive Konrad Curze with him. Sanguinius hoped to use Curze's prophetic abilities to determine what he was meant to do upon Davin. Sanguinius then commended a mass landing on the world, and the enraged Lion nearly ordered that Davin be subjected to Exterminatus regardless of Sanguinius' presence on it. At the last minute the Lion relented, and immediately was horrified by what he had nearly done. He realized that some power was attempting to force the Primarch's down the path to damnation, and followed Sanguinius down to Davin.
At Davin, Sanguinius was trapped within a portal and did battle with the Daemon Madail while Guilliman and The Lion desperately tried to reach him. A vicious battle erupted both on Davin and above it, in which Guilliman's acting flagship was destroyed in orbit by the Veritas Ferrum. During the battle at Davin's temple, Guilliman and the Lion managed to finally fight as brothers and together they brought down a massive Soul Grinder. Eventually, Sanguinius was able to escape and the Space Marine forces evacuated to space. Davin was destroyed, and with its anchor gone the Daemonic fleet vanished.
In the place of where Davin once was, a breach in the Ruinstorm was visible. The path led to Terra, but upon further study it became apparent that somehow Horus had foreseen this route and a large blockade was erected to block them. Guilliman and the Lion agreed to distract the blockade while Sanguinius and the Blood Angels made directly for Terra, for that was their destiny. The Lion expressed his desire to further distract the enemy by attacking worlds Horus had captured along the way to Terra, an act of vengeance he would come to relish.
Eventually the Lion made course for Terra but was too late. But hardly waiting things out.