Text 23

ye ’py anya-devata-bhakta
yajante shraddhayanvitah
te ’pi mam eva kaunteya
yajanty avidhi-purvakam

Translation

Those who are devotees of other gods and who worship them with faith actually worship only Me, O son of Kunti, but they do so in a wrong way. 

Commentary by Srila Prabhupada 

“Persons who are engaged in the worship of demigods are not very intelligent, although such worship is offered to Me indirectly,” Krishna says. For example, when a man pours water on the leaves and branches of a tree without pouring water on the root, he does so without sufficient knowledge or without observing regulative principles. Similarly, the process of rendering service to different parts of the body is to supply food to the stomach. The demigods are, so to speak, different officers and directors in the government of the Supreme Lord. One has to follow the laws made by the government, not by the officers or directors. Similarly, everyone is to offer his worship to the Supreme Lord only. That will automatically satisfy the different officers and directors of the Lord. The officers and directors are engaged as representatives of the government, and to offer some bribe to the officers and directors is illegal. This is stated here as avidhi-purvakam. In other words, Krishna does not approve the unnecessary worship of the demigods.

Commentary by Sri Visvanatha Cakravarti Thakur

In verse 15 you have mentioned the three types of worship, and then to explain the third type of worship (visva rupa) you have recited verses to illustrate it (verse 16-19).  Some others worship Indra and others as part of their practice in karma yoga (verse 20). Though they predominantly worship other devatas, they also are your devotees. Why do they not get liberation? For you have said, "They again take repeated birth and death (verse 21)" and "Men of small intelligence worship the demigods, and their fruits are limited and temporary." BG 7.23

This verse answers. Yes, they do worship me (te mam eva yajanti). However, they worship without any rules for attaining me (avidhi purvakam). Thus they return to this world.