The ordering of the flags is wrong. The --class flag is supposed to apply to the terminal emulator, not htop. You need to put the --class term before the -e flag, otherwise, your terminal emulator will simply treat the entire string after the -e flag (including the --class flag) as a command string to be executed by the terminal. In other words, if you put --class after htop, the terminal will assume that the --class flag is meant for htop, not your terminal.
No problem at all. I’m glad you managed to get it working.
Oh, yeah. xfce4-terminal doesn’t support the --class option. However, if you prefer to use xfce4-terminal to launch your floating htop window, I think the WM_CLASS can be assigned with xdotool.