They WELL-established Superman was all of those things before Supergirl ever showed up, and was the most beloved champion of Earth for the last 15 years, with monuments and parks built in his name and entire new government agencies started because of him, so this is a bunch of nonsense. Supergirl had to spend an entire season just getting out of his shadow and establishing herself as her own hero. Flash and Green Arrow haven’t known him long, but they both already look up to him (they’ve never really revered Kara like that - they treat her like their little sis). The one thing I’ll give you is the Legion, but I associate Supergirl more with them in the comics these days anyway with her headlining titles with them and spending time in the future with them, so that change actually kinda made sense to me. But more importantly to your point, that future where the Legion came from had been wiped out by the Blight causing a massive information blackout for years, and so they had no records of Superman - the only reason they knew about Kara was because of Mon-El, who accidentally time-travelled there (and was in love with her at the time). They stopped the Blight threat from ever happening, so that future no longer exists, and Superman can now be a known inspirational figure to them. And considering he already has a Legion ring in the Fortress, I’m sure that will likely be the case.