Basically Joselu is just a replacement option, Ancelotti's decision to play Joselu as a main player during pre-season cannot be separate from feasibility test considering he had just been brought in. Vinícius Junior's real partner up front is Rodrygo, coach Carlo Ancelotti is prepare the both for a new challenge for Real Madrid in the coming season without Karim Benzema.
For the reserve player level, Joselu isn't too bad in my opinion
When La Liga starts, maybe we will see the real performance of Real Madrid, at this time I agree that Ancelotti in friendly matches only tests the feasibility of players and try various experiments. As usual, coaches often do it to try different strategies. Actually you are right, Real Madrid signed Joselu only as a replacement option but if Ancelotti used a 4-3-3 formation maybe Joselu could have played as a centre-forward.