That's your assumption.
Since every other time this expression refers to Jesus gives further proof that Paul believed Jesus is YHWH here.
Simple.
You call this an assumption? Peter quoted directly from the same passage that Paul did and it's very clearly about YHWH. Nothing about calling on Jesus in all of Scripture to be saved.
Joel 2
28And it shall come to pass afterward,
that I will pour out my spirit upon all flesh; and your sons and your daughters shall prophesy, your old men shall dream dreams, your young men shall see visions:
29And also upon the servants and upon the handmaids in those days will I pour out my spirit.
30And I will shew wonders in the heavens and in the earth, blood, and fire, and pillars of smoke.
31The sun shall be turned into darkness, and the moon into blood, before the great and the terrible day of the LORD come.
32And it shall come to pass, that whosoever shall call on the name of the LORD shall be delivered: for in mount Zion and in Jerusalem shall be deliverance, as the LORD hath said, and in the remnant whom the LORD shall call.
Acts 2 (KJV)
17And it shall come to pass in the last days, saith God, I will pour out of my Spirit upon all flesh: and your sons and your daughters shall prophesy, and your young men shall see visions, and your old men shall dream dreams:
18And on my servants and on my handmaidens I will pour out in those days of my Spirit; and they shall prophesy:
19And I will shew wonders in heaven above, and signs in the earth beneath; blood, and fire, and vapour of smoke:
20The sun shall be turned into darkness, and the moon into blood, before that great and notable day of the Lord come:
21And it shall come to pass, that whosoever shall call on the name of the Lord shall be saved.