Krishna says something generous here. Those who worship other deities with genuine faith — they are actually worshipping me, even if they do not realize it. Because all deities ultimately lead back to the one source.
But the word 'avidhi-purvakam' adds a nuance. Their worship is 'without proper method' — not because their ritual is wrong, but because they do not see the full picture. They are reaching toward the sun but holding onto a lamp. The lamp is real, the light is real, but the source is far vaster than what they have glimpsed.
Krishna does not dismiss their devotion. He acknowledges the sincerity. He simply points out that knowing the complete truth makes the same worship far more fruitful.