Is it true that neuer can stand for both - and thus the only way to figure out if it's newer or just new is by context?
Yes, both sources are correct. The regular comparative is formed with the ending -er, so "neuer" is the comparative of "neu":
neu, neuer, am neuesten
Then we have the endings in the declension of adjectives, and yes, for the indefinite article, the nominative ending is also -er:
ein neuer Wagen
Yes, it sounds confusing, but we all will just have to live with it ;)
Yes, they can mean both. Now how to distinguish between them.
If an adjective stands in front of a noun it will get some sort of ending depending on case, preceding article and numerous. One possible ending is -er. The minimum ending is -e. There will never be no ending. So if you see "neuer" in front of a noun, the -er MUST be an ending and the word is hence just "new" (neuer - ending-er = neu). If neuer is not in front of a noun but rather a complement to a verb, then it means "newer" as it has no ending there.
Mein Wagen ist neu, aber deiner ist neuer.
If you have "newer" in front of a noun, then the basic adjective is "neuer". This will then get the appropriate ending leading to such beautiful words like... neuerer
Ein neuerer Wagen wäre ökologischer.
Side-note: I doubt that every German would notice if you said "neuererer" as long as you say it fast and with confidence. To some it might even feel right after you told them what you did.
"Neuer" has two functions:
- the undeclined comparative of "neu"
- The declined adjective that means "new" when used before a masculine noun when the preceding word does not already express the gender (eg "ein neuer Wagen" but "der neue Wagen" since "der" already makes it clear that we're talking about masculine gender). Compare "neue", "neues" for feminine and neuter respectively.
Since 1) is not declined, then you will decline it when needed, so eg you will say
ein neuerer Wagen
Ich habe einen neueren Wagen
etc. (but "mein Wagen ist neuer", however there's no ambiguity since the non-comparative form would be "mein Wagen ist neu").