In plain words
Even devotees of other deities who worship them with faith are, in truth, worshipping Me alone, son of Kunti, though they do so in ignorance.
Word for word
Even those who, devoted to other deities, worship endowed with śraddhā, they too, O son of Kuntī, worship Me alone, [though in a way] preceded by ignorance.
The commentators
Bhagavadgītābhāṣyam
ननु अन्या अपि देवताः त्वमेव चेत् , तद्भक्ताश्च त्वामेव यजन्ते । सत्यमेवम् —
येऽपि अन्यदेवताभक्ताः अन्यासु देवतासु भक्ताः अन्यदेवताभक्ताः सन्तः यजन्ते पूजयन्ति श्रद्धया आस्तिक्यबुद्ध्या अन्विताः अनुगताः, तेऽपि मामेव कौन्तेय यजन्ति अविधिपूर्वकम् अविधिः अज्ञानं तत्पूर्वकं यजन्ते इत्यर्थः ॥ २३ ॥
In plain words
But hold on: if the other gods are also you and no one else, then their worshippers are worshipping you and no one else. True — it is exactly so.
Even those whose devotion goes to other gods — devoted to other deities, that is, standing as devotees of other deities — offer their sacrifice, that is, they worship, and they do it with faith, carried by the settled conviction that it is so. Even they, Arjuna, are worshipping Me alone — without the ordinance. And 'without the ordinance' means: out of ignorance. That ignorance goes before their worship. That is the sense.
Close to the Sanskrit
But surely — if the other deities also are you alone, then their devotees too worship you alone. True, it is so —
Even those who are devotees of other deities — devoted to other deities, that is, being devotees of other deities — sacrifice, that is, worship, endowed with faith, that is, followed by the conviction that it is so; they too, O son of Kuntī, worship Me alone, without the ordinance. Avidhi is ignorance; they sacrifice with that as its antecedent — this is the meaning.