Your lawyer is right and wrong. In Pennsylvania, you can be convicted of driving under the influence with no alcohol in your system if there is a blood test that shows that you were high from narcotics at the time that you were driving.
You are correct though in that the bottom end of the lowest tier of the PA DUI chart is 0.08. However, you can be prosecuted under a legal theory of general impairment. This means you can be prosecuted without the introduction of chemical testing. This type of prosecution is very difficult for the district attorney. In a general impairment prosecution, I would look for more evidence than in a DUI prosecution with a blood alcohol test.
Examples of additional evidence would be a failed field sobriety test, a specific act of bad driving like an accident, alcohol in the vehicle in an open container, etc. These pieces of evidence could show that a person was driving under the influence with no evidence of the blood alcohol content. Whether these pieces are enough to convince the trier of fact beyond a reasonable doubt of your guilty is anther question.
If you are prosecuted under a general impairment theory, with no blood alcohol evidence, the "legal limit" doesn't really matter. Your lawyer is part right and part wrong.