Flak's essential in a balance fleet
There are cases when they are not efficient, you seem to forget the one who wrote the post does not seem to be an expert player (he can't defeat corvette swarms though I may be wrong, then I am sorry), and it needs some skill to use flak properly, balancing their numbers right, not to waste huge firepower if you build too much etc. I advice him only build flak if he has the sufficient money and can build them fast from2-3 factories. Flak only works if they are in larger numbers like 15-20 (or even some more) and their enemy is spread out (shooting at 4 directions grants max efficiency). They are medium against long range and are bad against anything else, only raise mitigation levels of enemy ships without causing real damage.. And if your opponent switches ship types to say bombers and titan/capital ships, there goes your useless flak.. In a close fight when you go to battle at the very start against corvettes pure light frig fleet is the best you can do, no need for researching and to have 2 military labs, and your factories build more firepower in lesser time. After a while thogh it may be necessary to build some.. Having some of them is always good, In multiplayer games where I faced corvettes I always had 10-20 (if heavy corvette fleet and strong eco of my own even above 20), they did good work on corvettes when I didn't want to build light frigates as they are dieing fast when facing long range.
if you don't have 60 within 10mins
Not everyone likes to build that much of light frigates, as TEC I always preferred long range (sometimes even as my first frigate built), and most of the times it worked, bought me enough time to properly balance my fleet against my opponent, countering anything he could send at me. Light frigates are stopped cold by long range frigates and cause you heavy economic damage (high losses), but for Advent they are excellent (Vasari is a bit different though I hate playing them in multi..). For TEC they can be good starters too, but only if your opponent is corvette-heavy or building corvette-light frig fleet (who has more light, he wins), but against long range they are ridiculously weak if you don't have extreme advantage in numbers.