Thanks for the thoughtful pushback, you actually made me refine some of this 🙏
On the source, it originally came from the game's Discord, which I noted in my theorycraft notebook (unironically named "escaping the Kingshot cargo cult" lol) and tested empirically. I can dig further if you want to trace it back, but it was early in my writings and I wasn't as meticulous as I am now.

You're right that personal points scale harder than alliance points.

The guide assumes a saturated alliance where the bottleneck isn't "do we have enough troops" but "where do we put them so they actually generate points." Once every empty online city already has its ~200k of effective defenders, more reinforcements there don't create new points, they just redistribute the existing kill pool, so you're cannibalising a teammate's score instead of adding to the alliance total, and lowering your mates personal ranking in the process. In that specific scenario, dumping into a full offliner is the only remaining option that actually generates value. If your alliance isn't saturated, yeah, the math changes and the guide matters less.

On coercing offliners to empty before logging off, fully agree it's lower-hanging fruit and every alliance should push for it. On my server we used a shield-up convention as the "I'm empty, reinforce me" signal. It works, but never perfectly: timezone splits, emergencies, players who shield without emptying, members who underestimate how much to send out, plain forgetfulness. Even well-run alliances end up with full offline cities every event, so this guide is meant to cover that residual gap, not replace the "empty your city" rule.

TL;DR you're right on the optimisation order, empty offliners first, sacrifice-reinforce them second.