We're native Russians and live in Germany for quite a while (17+ years). My children (8 and 5yo) were born here and we're raising them bilingual. They went/go to normal kindergarden/school and have natural exposure to the German language. At home, we speak Russian with children, unless there is a non-Russian-speaker present. The older son additionally goes to the Russian school on Saturdays. We often talk to our parents (who live in Russia) over videochat. Me and my wife are fluent in German, my wife is even Master of Arts in German language and literature studies). We parents also speak English and some French. A few years ago we even had a habit to speak a different language (Russian/German/English) each day, just for practice. We dropped the habit when we got children, completely switching to Russian at home.

I would say that our experience largely overlaps with what I read in the article. Raising bilingual child is hard, "truly bilingual" (whatever the measure for "truly" is) is even harder. But it is probably easier if you're living in a foreign country.

Judging from the older son - yes, his vocabulary in German is somewhat smaller compared to his German peers, but not significantly. His Russian vocabulary is probably also smaller compared to Russian kids, but more than that - it is different. He learned the language primarily from us parents, there are almost no Russian friends around, so his language is very adult, I'd even say academic. For instance, he normally does not use the word "круто", Russian equivalent for "cool", he'd rather say "remarkable". His grammar in Russian, is, however, heavily influenced by German. Prime example are reflexive verbs. For instance, for "I'm wrong" you'd probably say "ich habe mich geirrt" in German - literally "I have erred myself". Note the usage of the reflexive pronoun "mich" ("myself"). You don't use it in Russian as reflexion is encoded in the verb suffix itself - "я ошибся". But my son sometimes still uses the reflexive pronoun in Russian, saying something like "я меня ошибся" which is not correct and sounds pretty funny.

Over the years we've estabilished a small set of rules for language. Speak Russian in the family, unless a non-Russian-speaker is present. Answer in the language in which you were addressed. Do not mix languages. It seems to work pretty well for us.

The article seems to take mixing of languages quite easy. We see this as one of the biggest challenges. It is way too easy to start using German words where Russian translations are cumbersome or unusual. (Prime example is "Termin" - "appointment". In Russian there's just no good translation for this word. The closest is probably "назначенное время" - "the appointed time", but that has a completely different connotation.)
It may be not sound so bad if you mix Romance/Germanic languages, but if you mix Russian and German, the result is absolutely horrible. You end up with a language which neither "pure" Russian nor "pure" German speaker will understand. You'll need to know both languages to understand the mixture.

So this is probably the rule where we are most strict and persistent. Do not, never, mix languages. If you don't know a word, ask, we'll help. If it's too complicated, say it in German, we'll help to translate.

It is hard to keep the language and it is easy to loose it. We know families which lost Russian in the next generation. Their children understand some Russian but answer and German. I think this is a pity. Second language has great value, it takes so much effort to learn it as an adult, so it is unforgivable to loose this opportunity in the childhood.