Is it true that a fetus can only get infected via placenta? Is it correct to say that a pregnant woman can't transmit any infection bfore implantation?

No. A fetus can become infected in different ways. Through the bloodstream through the placenta. Also any infection in or nearby the uterus can be directly transmitted through a fetus. Common example is appendicitis which infects all surrounding tissue including often the uterus and fetus. Now a parent with an infection such as hepatitis or HIV can directly infect the egg and sperm prior to implanting.