Doesn't make sense and is therefore also not true. I'm working for one of those 'Asian' manufacturer of micro controller. Those are shipped neither via the UK nor via Rotterdam but always via air freight, a lot via Frankfurt but of course it depends on where the micro controller are used by the likes of Bosch, Continental and others. Not so much by the OEMs. Most manufacturing is done in Germany, but there is some manufacturing in France but also e.g. Ireland.
Those items have a far to high cost / weight ratio to send them via ship.
I work for a freight forwarder handling imports to Europe and fully agree with you. Reasons to import via another EU country are commercial or logistical, ie. a company in member state A has a supplier in Asia and they import all cargo first to their own country and then sell to their customers in member states B, C and D. This way they can either control the supply chain or get economies of scale logistically.
Which makes my point.
It is not just rules on taxation of imports. It is company law, financial law, ease of access, ease of setting up a business relationship.
I work regularly in the EU. I have a UK limited company. To set that up in the UK I need £150 and a UK registered address. In Belgium I need a degree and put down a €16,000 deposit.
Commercial law is entirely different in different EU countries. The UK has the most open commercial law in Europe and the least regulation over businesses.
To assume that a HUGE impact such as the UK, the 5th largest economy in the world, leaving the EU, would have little or no impact on the EU, even when the motor trade was heavily lobbying the commission to make an exception to vehicle parts, is a little disingenuous.
Ford manufactures 1.6m engines in Dagenham each year. It sold 155k cars in the UK in 2020. These engines will have up to 30 or 40 micro controllers in them.
To try and avoid rules of origin it is highly likely that EU vehicle manufacturers are trying to get more stock of micro controllers in order to fabricate out of the UK.
Now they are finding that supplier relationships, commercials and supply chain don't change in a few weeks.