It is (obviously) invalid to draw conclusions about the phone's antenna from comparing WiFi ad-hoc mode against WiFi infrastructure mode.
Even if it were valid, the antenna itself is not the only limiting factor.
- Modern 802.11 infrastructure modes (such as 802.11ac) deploy beamforming to extend ranges (some vendors support it already with 802.11n, but interoperability was poor).
- Modern 802.11 infrastructure modes deploy radio power management techniques to adjust for the desired range.
As opposed to that, WiFI Direct/ad-hoc may well require a fallback to older 802.11 transmission standards, not only limiting bandwidth, but also power management and beamforming abilities (in general or with specific hardware). Additionally, radio power management in ad-hoc networks is nontrivial, so even if it were implemented, that doesn't mean it would work well. With huge WiFi problmes in metropolitan areas already, the cheapest and safest approach is to not bother at all and reduce radio power to the minimum in ad-hoc scenarios.