A cleric currently has from 6 to 9 feats, so they can certainly choose to spend them on melee if they want. A cleric's feat supply is exactly the same as a barbarian or paladin's, after all. If DDO included
more worthwhile combat feats, then the extra slots of a fighter might be more of a benefit; but it doesn't.
Is a THF cleric behind a TWF fighter? Of course he is, just like a THF fighter is. Within the same weapon style,
currently the difference is the fighter has +1.5 damage from str, +6 from Specialization, and some unimportant crit-confirm. The cleric has +3 DF, meaning he's +4 per-hit behind the fighter. Assuming a typical 55 per hit, the cleric is 7% behind.
7% less DPS is not enough to compensate for some pretty awesome spellcasting, especially since the cleric's spells can boost his own DPS, one way or another (either by spells that damage monsters, or healing himself so he can keep fighting long after a fighter would have died or retreated)
Yes, it's true that Kensai will help give fighters a real melee advantage over clerics in terms of pure melee offense:
that's a good thing. Fighters need something to compensate for all the spells they don't get (and Will saves they fail)