It actually is possible to determine a world's seed just by looking at some screenshots, thanks to the power of modern graphics cards; for example, here is a thread where somebody was able to crack seeds within 8 hours on an average (for 5 years ago, now probably not even entry-level) GPU:
You can even download the program they made here (this one requires knowing the locations of structures though; I presume what they did in this case was actually recreate the terrain for an area and search for a match, which is also possible but much more computationally intensive; still, don't underestimate the power of modern GPUs; for example, the MD5 hash function, which is 128 bit, compared to only 48 bits for most of Minecraft's world generation (only the biome pattern uses the full 64 bit seed), is nearly useless for security these days as it can be cracked within seconds).
Of course, if you doubt it, you could try the seed yourself (I haven't but I believe it as I've seen others show it).
Yes, this is the correct id of the world he was using in his videos. For those of you, who are still not sure, that this is the correct world seed, check this article: https://olivias-blog.net/pewdiepie-minecraft-seed/ , there is a proof that this is the correct seed.