That is a true fact. And applies to most of them. Referring to that 80s-90s era, the majority of the big Sikh run tailor shops on Sukhumvit Road, between Soi 1 and Asoke, used to send all their tailoring work to a couple of double-wide shophouses at the end of Soi 4, on the right, just before the entrance to the tobacco monopoly. There were rows of sewing machines and fans spinning day and night. Presumably it was a lot of cheap, migrant labor doing the sewing. The tailor shops on Sukhumvit were/are really just glorified (high priced) fabric shops.
Not sure if that's still where all the cutting and sewing is done for the remaining tailor shops, but at that time I was told I could just purchase good quality fabric from Chinatown and take it to one of those shophouses to have it turned into a suit at very low cost. I never tried it out though.