Furthermore, enemy AI would be the key differentiator. In SMB1, Goombas walk left. In SMB3, Koopas can hide in shells, and Boos turn away when you look. In Mario NES 1.5, we would see the first "smart" enemy: a single Red Koopa that turns around at a ledge, or a Hammer Bro. that actually aims at Mario’s predicted position rather than throwing in a fixed arc. These are the kinds of incremental, "service pack" upgrades that characterize a 1.5 release.
Despite the name, it wasn't a "Mario game" but a general-purpose NES emulator. Using Mario's name was a common tactic for hobbyist projects to immediately signal their purpose to gamers. MarioNES 1.5