פירוש על קידושין 77:7

In this second statement by R. Assi in the name of R. Yohanan, the sage claims that kilayim is prohibited by the Torah outside of Israel. He expresses this by referring to the punishment for the transgression—a normal way for the rabbis to express the level of a prohibition. If the prohibition is considered to be from the Torah, then it is theoretically punished by lashes.
This again contradicts the baraita we saw earlier, which stated that kilayim outside of Israel is only a rabbinic prohibition.
The answer is that there are several different types of kilayim. One is the prohibition of planting seeds in a vineyard (Deuteronomy 22:9). This prohibition is only “derabanan”