I am pretty sure this is the case though. When Intel went LGA for their CPU's it put a lot of packaging costs off Intel's hands and onto the manufacturers. For Retail boards this isn't a huge deal but for OEM guys this was huge. AM4 was designed and specifically avoided going LGA at the behest of board manufacturers retail and OEM.Naturally, the display output lines conflicting with the x16 slot connectivity is also not true. I didn'T check but it is very unlikely, because you would need some switching hardware on motherboard for that - makes no sense really a mobo vendors would hate that because extra cost.
I tried to find a pin out but came up empty handed but every piece of information we have is that AM4 is heavily packed in pin usuage. This could be them being forward thinking and have pins that might not be in use now but will with Zen 2 and Zen 3. But still if you were creating a die that wasn't going to have support for those 8x PCIe lanes, would you continue to throw those PHY's in? Their absence doesn't mean that is sole reason why they have less PCIe lanes.