Weakness isn't always considered if a particular strategy (like Virizion/Genesect as you mentioned) is so effective, or if there isn't a popular deck that focuses on the type that the deck is weak to. In the case of this deck, the only Fire decks in the competitive scene are Flareon (inconsistent at the time of Worlds) and Pyroar (VirGen's absolute worst matchuo). Pyroar's hype also died down after many players took measures to counter it. Garbodor, Empoleon, and Accelgor (legal and popular at the time of Worlds) all give Pyroar a rough time, and they all had some presence at Worlds.
However, there are plenty of decks that do factor weakness into their equation. Seismitoad has a particularly hard time against VirGen, and even decks running Leafeon can give them a hard time. As a counter measure, these sorts of decks will run either Fire attackers, or Pokemon that aren't weak to their main attacker's weakness. Seismitoad / Pyroar is a combination that some players have embraced. In general, competitive players consider the weaknesses of their deck as a whole rather than the individual Pokemon in it, and guage how commonly played those weaknesses are. If a bad matchup doesn't see a lot of play, many competitive players will opt to "accept the autoloss," and focus on their deck being consistent in its strong points rather than try to improve the odds of something that still won't be advantagious even with techs.