As with always with these types of claims, the question is not whether it happens or not, but in what quantity. If 50-100 people or even a few thousands out of 328 million nation do this, is that really significant? It's like when some article says that "twitter is angry about something", and then it's actually just a couple of tweets from some nobodies.
I've been seeing a good market price right now for the bitcoin, I don't want to conclude that it was because of the stimulus checks that the american have received last week. But you are right, even if these checks are going to inject to the market it would just cause a small curve and that's it. I believe american that is investing in crypto is not spending their stimulus checks, most probably they are just spending what they kids want, the $500.
The problem here is that some less experienced people might start having unrealistic expectations about the market, they will start thinking that events like US stimulus can move Bitcoin's price significantly and predictablym which is really wrong.
Speculation these days compared to the past few years is different, the expectation towards the market is very high that even a single mistake will keep them away from the market.