I thought about this and well, it sounded either natural or suspicious based on what I wanted to hear... I actually started to think there wouldn't be a decisive difference between the pronunciation of "fontos" and a hypothetical "fóntos" word. Just like you can't really tell apart those who pronounce "utca" while thinking "ucca" in their head - and those who think of "utca" in their head and follow usual phonotactics. It's not impossible there would be some minor length difference or the ó version would sound more closed but nothing easy to perceive, even for natives. "o" and "ó" are mostly perceived as the same sound with different length and there is no such a thing as a 3 morae rhythmic unit. Both "fontos" and a hypothetical "fóntos" word would naturally have a 2 morae+1 mora rhythm, a 3 morae+1 mora pattern for "fóntos" would be like hypercorrect.
So at the end of the day, I don't think we (or I, at least) could even find out whether the two o's sound different regarding length or "shape". One thing is sure, it received a sentence stress which made the first o high pitch. This is unquestionable.
Köszönöm szépen! I had not known that “o and ó are mostly perceived as the same sound with different length;” rather, I had assumed a difference comparable to that between “a” and “á,” meaning that “o” would be pronounced as in the English “hot” (IPA: ɔ) and “ó” as in the English “mourn” (IPA: o or ʊ). So the difference is in length only, not in openness/closeness.
Thanks again for putting me on the right track.
Yeah nah, both are /o/, ó possibly a bit more closed among /o/'s. Apart from a-á and e-é, there isn't a big/consistent enough difference between the two variants - in fact, vowel length differences (sometimes even consonant length differences) are not unusual between speakers. "tűzi" is usually pronounced as "tüzi" when it's not the verb but the relational adjective, "szívat" as "szivat", but there is also "szalag" as "szallag" (although that annoys me a lot personally :D).