While I agree that "sprucing up a park" could be a sign of a potential sale; I do not think committing an expenditure of $150m a year indicative of someone preparing for sale as they would never get a return on that big of an investment.
A buyer is not going to pay a big enough premium for new rides to surpass the expense of building them.
It would be similar to buying a new car as an investment. It's just not going to pan out to the plus.
In the perspective of the additions we already know of:
Does Finnegan's, RMC Gwazi, Tigris, MMXX, cutback water coaster, and a new launched coaster at swo make the portfolio collectively worth $300m more? Will adding the 2021 projects make the portfolio worth $450m more? The answer is no. And that's the amount extra they'd need just to break even.
Even if they were planning to sell, nobody is going to buy with this many outstanding committed projects. They'd be inheriting contracts they didn't write along with all the risks (cost overruns, delays, etc) that come with any construction project. No reasonable board would sign off on that. So the comment that the stock price is inflated right now may be true; but can't be considered as part of a sale argument. Any potential sale is unlikely to happen prior to 2022 and you can't forecast stock price out 3 years in the future.
As a final thought, who would even be a potential buyer? Six flags is swamped with international projects and unlikely to want to double up on properties in regions they already compete in domestically. Universal is starting to build a third gate (4 if you count the waterpark) and have nothing to gain by adding SWO. Also, SEAS doesn't own any major IP that would be attractive to universal (or Disney, obviously). Cedar fair... maybe? But do they really want two parks separated by 1.5 hours? I'm not sure who else would be able to afford it.