It is not a comparison, GTAV has one of the best lighting systems in any game last gen. Quoting it cannot due to "GI" is just silly. If this was realtime and fully dynamic then I would see the issue, it is a fully baked system (so we have been told) so other than bandwidth/texture information having it fixed or cycle through is more of a development/time issue than a performance one.
Seeing how the game can barely keep 30fps and 900p like this, it seems to me like it's definitely a performance issue.
Not saying we're never getting any form of GI with real time day/night cycle (isn't Driveclub doing something of the sort?) but given how many current gen games seem to opt for an arbitrary shift in the cycle (Infamous SS, even Shadow of Mordor) it's probably the easiest trade off.
If that email to GiantBomb was true, further more, implementing this lighting has been a huge challenge for them, so they probably cut corners where it made the most sense.
And the real time day/night cycle is something most people didn't even notice was there.
There should still be atmospheric changes in real time though, that would change the lighting.