I've heard this too, but I am being told that it may be that there is a varied amount of oestrogen hormone in water and this can give a male the appearance of a female!!
Not sure how true it is, but it sounds kindda logical to me.
Late developing male swordtails tend to be more hardy and robust then their other male cousins. They also apparently produce more fry with each impregnation!!
I had what looked like a female...full body with a dark gravid spot but it chased both male & female...I couldn’t see a fin but eventually it developed a small gonopodium.