There are multiple 'h' sounds, which they claim one of those sounds very similar/was used for the same purpose as the vowel a, and morphed into that vowel. Which doesn't fully make sense to me that they don't just call it an a.I thought the e and the o sounds were the only real vowels used in Proto indo European language? There was an a vowel sound? Hmm. Been a minute (more like 30 years) since Iāve read up in it, so likely weāve learned more about it.
That said, we're talking about a language that was around for thousands of years. People trying to definitively state stuff like it only had these 2 vowels because that's what we found on some hittite excavations (which used either a sister or daughter language), seems like an act in futility to me. It'd be like someone coming about and either A: claiming that germans only have these letters and sounds they use, because they have documentation that's what americans used (if sister language), or B: Same thing but swedish and old norse.
I am far from an expert though - last class I took on it was almost 10 years ago. Just something about the study always seemed odd to me when very specific things are claimed, like 'only 2 vowels'.