Nowhere does that suggest Cleveland is the reason for Broncos not making the Hall of Fame. The team didn't have a great relationship with the people that run the Hall itself until Mike Holmgren ran the team and made that a priority. I really don't see any way that there's a bias for all of the voters when only one person that votes for the committee is affiliated with the team, so I'll chalk that up as a nonsense conspiracy.
Four of the remaining 15 are Broncos (Davis, Atwater, Mecklenberg, and Lynch). I'd agree with the notion that you also suggested where they don't have a ton of household names outside of players they've had since the 80s (Elway, Mecklenberg, Atwater, Rod Smith, Terrell Davis, Sharpe, Zimmerman, Nalen, Champ Bailey). There aren't many well-known players from before then, which would be the choice of the senior committee that gets just two selections a year to make that even harder. They'll get more players now because those guys have retired and are now up for enshrinement. Elway retired way earlier than the rest of his teammates, so that's one reason for the disparity there.