Is the maximum temperature 85°C, or 245°C?

Long story short, I have to stick my project in the oven (reconditioning a BME280). It already has a Photon soldered to it and I don’t really want to have to desolder it. I checked the Photon’s solder reflow specs, and it says it should be good up to 245°C, whereas I’m only baking at 150°C for 2 hours, so I thought it should be fine.

But then I read the Photon’s max specs, and it says it can’t even be stored at a temperature higher than 85°C. So… will it be safe at 150°C for 2 hours, or not?

You should look at the Photon datasheet, particularly the section on reflow soldering which is where the 245 degree C number is from:

They recommend holding the device for 60-120 seconds (1-2 minutes) between 150-200 deg. C so that amount of time at that temperature is clearly fine.

I don’t think you should bake either the Photon or the BME280 for 2 hours at 150 deg. C.

Well yeah that’s what I’m looking at, that’s where I got these numbers from. But that datasheet conflicts itself, because it later says “ABSOLUTE MAXIMUM RATINGS: +85 °C”, which is why I was a little confused.

Peak reflow temperature is just that,an absolute maximum temperature a device can reach for a very short period once (or maybe twice on a double sided board but not with a module like this) as it goes through the Reflow oven.
The BME datasheet tells you to recondition @ 120C not 150 and I would think most assemblies would be OK with that.

normal environmental operating conditions vs soldering. most silicon normal operating temp is max 70C (commercial) - 85C (industrial)

you could always insulate the board except for the bme280. heat the board to 100C for 20 min and then use a hot air gun on the bme280. apply ~300C until you see the solder shine.