What were the circumstances? They don't really matter, but it would shed some light as to why he was getting flamed. Legendaries aren't nessecarily stronger than ordinary Pokemon. Regigigas is not the best choice of Pokemon to use since it has the ability Slow Start, so there are ordinary Pokemon that can perform better than it, so just because it is a one-of-a-kind does not mean it is stronger, so that excuse can't work. The flamer is probably under the impression that all legendaries have better stats for all the stats then all other Pokemon when that just is not true. There are many Pokemon that have higher HP, Att, Def, SpAtt, SpDef, and Spd base stat them Mew, a legendary that is perceived to be really good by people who tend to flame people for using legendaries. The flamer(s) were/are probably ex-Pokemon "fans" that only played the first generation (Genwunners, as I've seen them called) and think that they know enough about Pokemon to tell current players what to do such as giving Hyper Beam to everything (because
that makes sense). There seems to be a lot of those people, especially on websites that have little to no comment moderation, so the self-entitled idiots just voice their opinions without thinking about whether they are making sense or not.
I'm agreeing with you, but you made a few mistakes I want to correct for you:
legendary such as Cobalion, Rotom, Virizion, Zapdos, etc.
Rotom's not a legendary since it can be bred to produce new Pokemon of the same evolutionary tree.
Pseudo legends don't even EXIST. It's just a term made up by a 30 year old in his grandmother's basement.
The term itself is not official, but there is a Pokemon in each generation (2 in Hoenn's case) that follow a set of criteria that make them almost "legendary" in strength, hence them being called "Psedolegendaries" which means "fake legendaries". These Pokemon are Dragonite, Tyranitar, Metagross, Salamence, Garchomp, and Hydreigon. They all have an immunity in one way or another, all have a three-stage evolution, require 1 250 000 exp to reach level 100, they are all evovled into at high levels, it takes 41 egg cycles to hatch their eggs, all their first forms have a BST of 300, and, most importantly, they all have a BST of 600.