Do Americans pronounce ‘I’ll’ as ‘all [ɔ:l]' as an American ESL teacher says, frequently?
I'll is not really pronounced the same as isle or aisle in American English in normal speech. When said to read "I'll" in an isolated position some people might pronounce it as isle or aisle. In reality it is always pronounced as if it rhymes with doll in American English. It is a similar situation with the word "to." If isolated (or in quotation marks like I just did) Americans will pronounce the same as too or two. However, it is almost always pronounced with a schwa unless it's at the end of a sentence, for example: "I don't want to." Regarding the pronunciation of "all" and "I'll" being the same, in the General American accent, they are not. I have seen the Rachel's English videos before and I would have to say that is the main problem that I have with her. She teaches the cot-caught merger which is not standard. So, in conclusion "I'll" is not pronounced like "all" or "isle/aisle" in the General American accent. It is pronounced like "doll" but without the d.
In American English, I'll [aɪl] is usually pronounced as "al," much like "isle." A common example would be I'll go to the store later. Which sounds like Al go to the store later. When the word's length is dragged out, it has a more southern accent, while when it is short and crisp, it is more like a northern accent.
Note that it is not pronounced "Ahh-l".