I'll say your sentiment is mostly correct, but it probably wasn't a matter of minutes. To use an existing DF setup on 315 MHz means changing your LO (local oscillator, aka your frequency source) and filters at a minimum. If you're using a digital/programmable oscillator, then changing the LO is easy. If you're using a fixed analog oscillator then you are probably ordering one because no ham has a 315 MHz crystal lying around.

Any well designed DF is going to have filters to improve the signal, and even a single pole 440 MHz high-pass or band-pass filter will reject nearly everything at 315 MHz. If the DF is SDR based it may be using digital filtering, but it's far more likely to have analog filters, so you would either need to buy or build new filters. Buying filters that you'll probably never use again seems like a waste, so they probably would wind their own inductors by hand and solder the components themselves, which puts the filter price in the $1 range instead of the $100 range.

Even then, your 440 MHz antennas are poorly tuned at 315 MHz so any self respecting ham would have built some new ones out of a coat hanger at minimum.

So it probably would take at least one day for an experienced and motivated ham to make their existing DF equipment into a workable solution.

Yes and no. I usually use the body-shielding technique: Attenuate or reduce gain until the signal is faint. Hold the receiver against my chest and turn in a slow circle until it disappears entirely. Now I know the transmitter is behind me. It's crude, but expedient, and fairly effective.

This has the advantage of working with any receiver type, any antenna (it's actually particularly good with the otherwise-useless rubber-duck that cheap HTs ship with), and any signal with a wavelength longer than my torso is wide, -ish. It falls off gracefully at the limit there.

Would better gear produce better results? Sure, you'd almost certainly beat me to the fox if you had that stuff sitting around. But I'll be knocking on the guy's door while you're still soldering.

Incidentally, anyone who's been playing with TPMS decoding probably has a 315MHz-specific filter kit sitting around. It works okay-ish without that, but it gets truly creepy when given a good filter and antenna. SAW cans for 315 and 433 are about $4/ea.

I concede. Some people could have found this guy in a few minutes :)

I may have fallen into this trap: https://xkcd.com/974/