Yes, the sentence is grammatically correct.
**Meaning:**
The speaker is expressing that they were genuinely (using the informal term "legit") monitoring something, likely a count or statistic, and they were present when that count reached 200,000.
**Possible rephrasing:**
- "I was actually keeping an eye on it😄. I was there when it hit 200,000🎉."
- "I was genuinely tracking it😄. I witnessed it reach 200k🎉."