Hieroglyphic script is not only pictographic. Some symbols represent syllables. The famous Rosetta Stone is how we can learn how to pronounce their names: the same stuff was written in multiple writing systems, including the Greek alphabet.

Additionally, many of the names commonly used in English are Greek names (for example, Apophis is the Greek name for Apep). This implies that there are Greek sources for them.

I would be surprised to find out that there were no sources in Coptic on the topic of Egyptian polytheism. The Coptic alphabet is based on the Greek alphabet.

Furthermore, since Isis, in particular, became popular in the broader Roman Empire (I consider this to be common knowledge, if asked for a source), I would guess that there is documentation in Latinfor this reason.