The Hebrew phrase malak YHWH can also be translated “an angel of YHWH” or “a certain angel of YHWH” (depending on the context).

The NET Bible comments on Genesis 16:7: “Some identify the angel of the Lord as the preincarnate Christ…However, it is more likely that the angel merely represents the Lord; he can speak for the Lord because he is sent with the Lord’s full authority. In some cases, the angel is clearly distinct from the Lord (see Judg. 6:11-23). It is not certain if the same angel is always in view. Though the proper name following the noun angel makes the construction definite [grammatically], this may simply indicate that a definite angel sent from the Lord is referred to in any given context. It need not be the same angel on every occasion. Note the analogous expression ‘the servant of the LORD’ which refers to various individuals in the OT.”

Similarly, in Deuteronomy 15:17 God commands masters to take an awl and put it through the ear of a servant. The Hebrew here literally says “the awl,” but are we to suppose there was one specific awl that was kept since the time of Moses and used all the time?

Of course not.

Lastly, the Bible speaks of plural “angels of God” (Gen. 28:12; 32:1); these are clearly the same as angels of the LORD (see Ps. 103:20).