"Johnson selbst erklärte einem Journalisten gegenüber, er vermute Fidel Castro hinter dem Attentat. "
Ist "vermute" in Ordnung hier?
Ich hätte "vermutete" erwartet.
vermute is not present tense here, it's subjunctive 1 (Ger.: Konjunktiv I) because it's reported speech. It shows that what is said (that the journalist suspects Fidel Castro to be behind the attack) is the words of the speaker/writer themselves but they are quoting what somebody else said.
The subjunctive 1 is often extremely similar to the present tense. For vermuten for example, all forms are the same except the third person singular which doesn't have the final -t in subjunctive.
To be fair, subjunctive, especially subjunctive 1, is not very common in everyday speech anymore, which is why these conjugation patterns are the only ones even German children are actually taught in school to make sure they know how to use them. Fortunately the subjunctive 1 is very regular: Its forms are always derived from the present/infinitive stem (in this case arbeit-) and have the endings (ich) -e, (du) -est, (er/sie/es) -e, (wir) -en, (ihr) -et, (sie) -en (in the case of sein the -e is deleted where there is no consonant following: sei instead of seie, but apart from that even sein follows the pattern regularly: (ich sei,) du seiest, (er/sie/es sei), wir seien, ihr seiet, sie seien).
Hope that helps.