update: There was a helpful edit in the queue and I'd accepted it. I then added a further helpful edit. I think this can now be reopened and the blockade on answer-posting can now be lifted.
There are several reasons for closing the question for improvement, but perhaps the OP was overly vilified prematurely.
And if the mention of the name of a company is felt to be problematic (it really wasn't promotional at all in this case, you folks over-reacted) then just ask the OP to remove the company name or make a quick and helpful edit yourself.
Closing and especially the answer prevention that it what closing does is heavy-handed when a tiny edit can fix that problem!
There is some physics here, let's be careful.
They didn't see gravitational waves, but perhaps they saw gravity waves.
Gravitational waves, the wiggling of spacetime due to huge things like stars and black holes accelerating are not going to be detected like this.
But there are also Gravity waves familiar to Earth scientists!
And low frequency, wave-like variations in local gravity are certainly possible.
Their period was about 9 hours, and an accelerometer might see a 24 hour periodic variation in acceleration as well.
Right now these are measured as fairly fast impulses following seismic effects, but this is just the beginning of a new field in Earth observation science so let's be careful not to rule out something without careful consideration.
See my summary in Seismic amplitude distribution and see
There is certainly an interest in long period gravitational waves, cf. What produces gravitational waves with “periods between about 100 - 8000 seconds”? which piqued their interest, but these are not observable with conventional accelerometers
Or Tidal Effects!
If there was any chance their 3$\times 10^{-5}$ Hz peak was consistent with 2.3$\times 10^{-5}$ Hz then a 12 hour period would be consistent with tidal effects https://astronomy.stackexchange.com/a/35562/7982