I've heard that it is really common for English speakers to work professionally in Poland, is this true?
Yes and no.
Generally, Polish companies are eager to hire Polish speakers who know English well (generally the more fluent the better). But, they're really not in the market for English speakers who don't know Polish (the phrase 'more trouble than they're worth' comes to mind). There is a greater likelihood of an English speaker who knows Polish language and customs well being hired but they'll be paid (and treated) like native Polish staff which is carries its own problems.
The English monolinguals in Poland are usually sent there by non-Polish companies operating in Poland and there may be staff around to smooth the way for them in various areas (which the foreigners in question may or may not realize).