Why would they be? Has the Japanese PM found a way to reduce the importance of geography for trade? Didn’t he say that the UK would no longer be the gateway to Europe? So what he is saying is “we can have a trade deal but obviously it won’t be as good as the one you have with the EU because you will lose all our EU trade”
I love how getting a deal not quite as good as the one the EU has is now a cause for celebration. “Haha things are only going to be a little worse rather than loads worse. PROJECT FEAR LOSERS”
The EU won't like it because it shows it is possible to get trade deals when outside the EU. And unlike remainer predictions of 'if we're lucky we'll get a deal with Senegal (replace with a small African country of your choice)', this would be a significant deal. Nobody has said one single deal would replace being in the single market. It's a collective of deals that will do that. That other trading partnerships are willing to do a deal with the UK somewhat weakens the EUs hope that we might be forced into staying in the single market and customs union if we couldn't find trade deals elsewhere.