This is very interesting, it is suggesting that BTC may bottom any moment and someone is planning a huuuge buy.
it could also mean that exchanges are locking up their real fiat into the tether fed reserve. and getting tether bearer bonds in return
meaning its not a buy thats about to buy coin. but an exchange just swapping real fiat for tether so they can arbitrage with othr exchanges in the background = no change
i seriously doubt that an exchange owner would bother with arbitrage trading between exchanges since the profit is already too small and most of the time near impossible to be any. there are much easier ways of earning money with the money you have including pumping the coins listed on their exchange, investing in some other altcoin and pumping it on some other exchange,...
Even though everything is possible, I'm inclined to think that its' not money from other exchanges. Exchanges are so rich by now that they would probably not waist energies for adding some little gains.