Reading this section of BIP144, I noticed the followng statement:
Parsers supporting this BIP will be able to distinguish between the old serialization format (without the witness) and this one. The marker byte is set to zero so that this structure will never parse as a valid transaction in a parser that does not support this BIP. If parsing were to succeed, such a transaction would contain no inputs and a single output.
And this is in fact confirmed by this line in Bitcoin Core which is executed for every transaction found in a block during the
As far as I understand, this should mean that old clients that see a block containing a SegWit transaction would consider that transaction invalid and hence discard the whole block. Is that correct? What am I missing?