You've been reading too much isekai dude... Why would you make the conclusion "we must be in a different world" when you see 2 species on the same island when they're normally just across a narrow strait?
His handkerchief was still damp on page 10.
Presumably that's when our MC suspected teleportation.
Everyone talking about the knife, but nobody is talking about the phone compass app that is useless without electronic signals, like cellular or GPS bands.
Why? A modern phone's digital compass is actually sensitive to the magnetic field unless really cheap.
On the other hand, GPS is worth mentioning. Not having Japanese QZSS satellites in the sky (when the manga's characters are outdoors) could be a clue that they're not in Japan. Additionally, not having any of GPS / GLONASS / BeiDou / Galileo almost immediately spells “isekai” because the other two most possible causes are
① time travel on Earth (and not too far because tigers and chipmunks are still able to be recognized),
② some catastrophic electomagnetic event (such as a planetary-sized solar flare, or a global thermonuclear war, or a gamma ray burst from the outer space) that could fry most of the navigational satellites at the same time.