Obligatory, since the opposite is being assumed in a few places: Agrarian Idyll is, by default, vaguely anti-environmentalist. Its core mechanic is to cause you pave over more nature in order to get the same housing (via lower per-city housing), or to get the same building slots (via needing 4 agri-districts to get one slot).
Fields of crops may literally be the color green, but they're not "green" in the environmental sense: they're an environmental hazard cocktail of pesticides, fertilizer runoff, and wildlife eradication to protect crops from pest losses.
The one thing Agrarian Idyll definitely is, is anti-urbanist. And urbanization is (generally*) environmentalist: better to house your citizens in high density housing with efficient transit than to spread that same number of people out over a larger area, paving over more of nature to so do. Ex. 10 acres could either hold 20 single family homes, or 2 apartment buildings housing those same 20 families and 8 acres of woodland. The latter both uses less resources and just literally preserves more space for the actual natural environment.
It also incentivizes you to clear blockers for more district space and agri-district capacity, which is the opposite of what Environmentalist defines as "preserving nature". Though that connection is quite a bit weaker.
*with the notable exception of an ecu, though even that's debatable: by default an ecu crams 5 planets worth of housing and 3 planets worth of jobs onto a single planet. One could argue that having 1 ecu and 2 completely undeveloped planets is theoretically more environmentalist than fully industrialized forge worlds, but that doesn't prevent the horrors of ecocide for the unlucky ecumenopolis planet.
You could absolutely have a headcanoned solarpunk Agrarian Idyll empire. The mechanics aren't so explicitly anti-environmentalist that it's incompatible. But it's not what the mechanics imply by default.
This is all sort of orthogonal to the thread topic, though.
I, too, am in favor of allowing AI empires to research Anti-Gravity, and instead directly barring them from taking the Arcology Project (and, possibly, restoring ecus from relics too).