Acknowledge does not equal recognize. You can acknowledge someone you don't recognize. There are definitions for acknowledge that start with "recognize" but the following words are part of the definition and most of the definitions involve things, when a person is involved definition 2 or 3b is involved. https://www.merriam-webster.com/dictionary/acknowledge
So you can acknowledge someone's presence, by taking notice of them or acknowledge somone's rights or ideas by agreeing that they exist.
"Ellos son difíciles de reconocer. " Translation:They are difficult to recognize.
"Ellos" is never a direct object. It is a subject pronoun or could be used in a phrase such as "para ellos".
A direct object pronoun would be me/te/lo/la/nos/los/las/vos.
"Reconocerlos" would be a reflexive verb form, but I do not believe "reconocer" is being used reflexive here. Not sure of the technical verbage for it's use here, though. Maybe someone else could comment.
Temporary vs. permanent is not a good way to think about ser vs. estar. It is far too simplistic and does not capture special exceptions. Instead, think of estar as a state of being and ser as a characteristic. Professions, including being a student, use ser. So, soy un contador and estoy enfermo are two examples.
I'm not native English speaker either (Swedish), but acknowledge has a totally different meaning. Acknowledge would probobly better be translated to Spanish with words like 'admitir' or 'agradecer' or 'confesar' or 'saludar'. As you can see, those words has something more included than the original sentence, wich kind of says that 'you saw them and remember who they are'.
I have to correct myself. 'They are difficult to recognize', means something more like that 'you would have a hard time to spot them and remember who they are'... 'They are difficult to acknowledge' would probably mean something like that 'they are difficult to admit' which makes no sense... Can someone native English speaker clarify a bit?
"To acknowledge" would be "to notice" them, but that doesn't mean that you know who they are. So, you know they exist and maybe you might know that they want to talk to you, but if you are well-known, it is possible that they know who you are but you might not know who they are. So if you know that you made a mistake, you agree that the mistake exists and that is when it can mean that you admit you made a mistake. https://www.merriam-webster.com/dictionary/acknowledge
I had to translate Spanish to English for "Ellos son difíciles de reconocer." I put "They are difficult to recognize." and I was marked wrong and told that the answer is "They are difficult to recognize." This was not a multiple choice, but I bet there is a multiple choice in which there are two exact copies of the sentence and one is right and one is wrong and it is affecting all the exercises. I could not report this properly as they only offered three choices not including this, so I reported that there was something wrong with the Spanish just to get them to notice this sentence.
It is different, because "ganar un torneo" is the subject and "dificil" is the predicate adjective which describes it.
This one has "ellos" as the subject and "dificiles" describes the subject. Now the infinitive is not used as a noun and in Spanish it needs a preposition to relate it to the word it describes.