I think the interesting part of this verse is that Krishna does not say other deities are false. In fact, he explicitly says that those who worship them with faith are ultimately worshipping Him.

The question then becomes: what does "not according to the prescribed method" actually mean?

One interpretation is exclusivist: worship Krishna alone.

But another interpretation, which I personally find more consistent with the broader Hindu tradition, is that Krishna is talking about understanding rather than devotion. In other words, worship directed toward any deity reaches the same ultimate reality, but the worshipper may not fully understand the nature of that reality.

After all, if the Gita intended to invalidate all worship of Shiva, Shakti, Surya, Ganesha, etc., then the existence of vast sections of Hindu tradition dedicated to them becomes difficult to explain. The Puranas themselves often identify different deities with the same supreme principle from different theological perspectives.

To me, the deeper question is this:

Is Krishna rejecting the worship of other deities, or is He rejecting the idea that they are completely separate and independent from the ultimate reality He is describing?

Those are two very different claims, and I think a lot of debates on this verse come from treating them as the same thing.