This can't be universally true. There are items of which there are very, very few Genuine ones. Say the Red Faction: Armageddon hammer - 200 something people got that. I know a guy in real life who traded it for a regular Batter's Helmet and flipped out when he was told how rare the hammer is.
You missed the "desirable to use" part of my quote. Meaning that it's a sought after item by
players and not just traders. The Machina, specifically a Strange variant which has kill counters, is one of the most popular weapons for players who play Sniper because of the bonuses it has.
The Maul from Red Faction is not that exciting to players because the Homewrecker is identical in terms of stats, and even the Homewrecker is not commonly used (the Degreaser + Axtinguisher combo is far more popular for Pyros who are decent). There is no Strange Maul anyhow.
Current Values:
Genuine Maul 7-9 Keys
Genuine Machina 2-2.33 Refined Metal (worth very little because of how many copies of DX:HR were preordered)
Strange Machina 13-14 Keys
(Keys cost $2.50 from the MannCo Store)
So that uber-rare Red Faction Maul is in fact worth less than a Strange Machina, which are far more common in terms of sheer numbers.