a single 18650, 3400 mAh cell costs between S$ 12 and 20 in Singapore
That's way overpriced, even if you are talking about prices in Singapore dollars. Panasonic doesn't sell cells directly to consumers, but an indication of their true value is that you can easily find e.g. 3.4 Ah ncr18650b cells from shops for vaping and flashlight enthusiasts for about 5 (US) dollars a piece, with battery manufacturers paying even less as they source the cells in bulk directly from the manufacturer. A 3s3p pack of them would cost about 45 dollars, plus the price of the case, labour and protection and balancing circuitry.
Radio control hobby shops also sell cheap high discharge rate batteries to consumers. For example, you can buy a 3s 2200 mAh 20C pack for 10 dollars, 5 of which in parallel would yield a 3s5p 11 Ah $50 pack.
I'd say it's quite probable that that pack is legitimate. However, there are many scammers in the world that count on you not having a charge counting charger. Beware of anything suspiciously cheap, or with "fire" in the brand name, you can bet that they lie about the specs.