Flexible in-office hours is a subset of unlimited PTO. Where there is unlimited PTO there is also flexible in-office hours by virtue, so, yes, technically you can have both. If you have unlimited PTO you can't not have flexible in-office hours.

However, if there is a flexible in-office hours policy in force, that does mean you can't have unlimited PTO. The additional restrictions placed on flexible in-office hours – those which narrow it to being a subset of unlimited PTO – would violate the grander unlimited PTO.

If an organization claims to have both a flexible in-office hours and unlimited PTO policy, there is a contradiction. While you can pretend to have both, you don't really have both. One ultimately has to supersede the other.