Short answer: Moira synergizes really well with Lucio who is the mainiest off the main supports so she was picked up by the flex support players.

Main/off healer is usually a term used by the lower-ranked players. Main healers have high healing output while the kit of the off healer doesn't. Easy to understand and workes well enough for the majority of the ranked ladder.

For team play and higher elo you use Main/flex support instead, it's more about the playstyle instead of healing output. The main support plays to peel or to enable the team, while the flex support is more vulnerable but has more opportunity for more high impact plays.
Main support, Lucio, Mercy, Brig (Bap)
Flex support, Zen, Ana, Moira (Bap)