Yes if you search youtube you can find people showing how to boil water inside a plastic bag. Typically they show this in survival situation scenarios. You apparently can also do it in a wooden container if done right.

But as to your question wouldn't they need to have pots? Well I know we don't have evidence of that but it wouldn't surprise me. Another method show for survival shows a person taking a fallen tree and building a fire by it. Then you place some hot burning pieces on top of the log. Keep adding them and burning on top of the log until it burns a bowl sized indentation. Then you take a rock or stick and scrape the hole and get it clean sort of. Then you put water into that and if no container to carry water is shows soaking a shirt and carrying it that way. After the large bowl sized hole is filled with water you take a few rocks that were sitting in the fire and drop them in. You will be amazed how fast it will boil the water. This is done to allow you to drink from a potentially unsafe water source.

I guess what I am thinking is that there are probably dozens of ways they could have achieved it. Ways that with our knowledge of today escapes us but to them it was common knowledge. If I had to take a guess they would have used rocks and use a large flat rock and encircle that rock with rocks making a pit or rocks then covered the sides with dirt. Then dug a hole under part of the large flat rock and made fire under it. This primitive pot would not work well at first but my guess is that as fat melted and oozed into the cracks it would eventually seal and then would work very well to boil things. Anyways just fun to think about I know very little about the time period.