Much of the ECUs OEM are purchasing from Tier 1 suppliers (Denso for example - a Toyota spinoff) who are directly hit by IC shortage, so that's why they are talking about overall supply. Also Japanese OEMs and suppliers tend to favor ICs from Renesas, and Renesas has been hit particularly hard this year [1].

[1] https://www.renesas.com/us/en/about/press-room/notice-regard...